Project for profiles and defaults for libvirt domains

[Martin Kletzander]

Hi everyone!

First of all sorry for such wide distribution, but apparently that's the
best way to make sure we cooperate nicely. So please be considerate as
this is a cross-post between huge amount of mailing lists.

After some discussions with developers from different projects that work
with libvirt one cannot but notice some common patterns and workarounds.
So I set off to see how can we make all our lives better and our coding
more effective (and maybe more fun as well). If all goes well we will
create a project that will accommodate most of the defaulting, policies,
workarounds and other common algorithms around libvirt domain
definitions. And since early design gets you half way, I would like to
know your feedback on several key points as well as on the general idea.
Also correct me brutally in case I'm wrong.

In order to not get confused in the following descriptions, I will refer
to this project idea using the name `virtuned`, but there is really no
name for it yet (although an abbreviation for "Virtualization
Abstraction Definition and Hypervisor Delegation" would suit well,
IMHO).

Here are some common problems and use cases that virtuned could solve
(or help with). Don't take it as something that's impossible to solve
on your own, but rather something that could be de-duplicated from
multiple projects or "done right" instead of various hack-ish solutions.

1) Default devices/values

Libvirt itself must default to whatever values there were before any
particular element was introduced due to the fact that it strives to
keep the guest ABI stable. That means, for example, that it can't just
add -vmcoreinfo option (for KASLR support) or magically add the pvpanic
device to all QEMU machines, even though it would be useful, as that
would change the guest ABI.

For default values this is even more obvious. Let's say someone figures
out some "pretty good" default values for various HyperV enlightenment
feature tunables. Libvirt can't magically change them, but each one of
the projects building on top of it doesn't want to keep that list
updated and take care of setting them in every new XML. Some projects
don't even expose those to the end user as a knob, while others might.

One more thing could be automatically figuring out best values based on
libosinfo-provided data.

2) Policies

Lot of the time there are parts of the domain definition that need to be
added, but nobody really cares about them. Sometimes it's enough to
have few templates, another time you might want to have a policy
per-scenario and want to combine them in various ways. For example with
the data provided by point 1).

For example if you want PCI-Express, you need the q35 machine type, but
you don't really want to care about the machine type. Or you want to
use SPICE, but you don't want to care about adding QXL.

What if some of these policies could be specified once (using some DSL
for example), and used by virtuned to merge them in a unified and
predictable way?

3) Abstracting the XML

This is probably just usable for stateless apps, but it might happen
that some apps don't really want to care about the XML at all. They
just want an abstract view of the domain, possibly add/remove a device
and that's it. We could do that as well. I can't really tell how much
of a demand there is for it, though.

4) Identifying devices properly

In contrast to the previous point, stateful apps might have a problem
identifying devices after hotplug. For example, let's say you don't
care about the addresses and leave that up to libvirt. You hotplug a
device into the domain and dump the new XML of it. Depending on what
type of device it was, you might need to identify it based on different
values. It could be <target dev=''/> for disks, <mac address=''/> for
interfaces etc. For some devices it might not even be possible and you
need to remember the addresses of all the previous devices and then
parse them just to identify that one device and then throw them away.

With new enough libvirt you could use the user aliases for that, but
turns out it's not that easy to use them properly anyway. Also the
aliases won't help users identify that device inside the guest.

<rant>
We really should've gone with new attribute for the user alias instead
of using an existing one, given how many problems that is causing.
</rant>

5) Generating the right XML snippet for device hot-(un)plug

This is kind of related to some previous points.

When hot-plugging a device and creating an XML snippet for it, you want
to keep the defaults from point 1) and policies from 2) in mind. Or
something related to the already existing domain which you can describe
systematically. And adding something for identification (see previous
point).

Doing the hot-unplug is easy depending on how much information about
that device is saved by your application. The less you save about the
device (or show to the user in a GUI, if applicable) the harder it might
be to generate an XML that libvirt will accept. Again, some problems
with this should be fixed in libvirt, some of them are easy to
workaround. But having a common ground that takes care of this should
help some projects.

Hot-unplug could be implemented just based on the alias. This is
something that would fit into libvirt as well.

========================================================================

To mention some pre-existing solutions:

- I understand OpenStack has some really sensible and wisely chosen
and/or tested default values.

- I know KubeVirt has VirtualMachinePresets. That is something closely
related to points 1) and 2). Also their abstraction of the XML might
be usable for point 3).

- There was an effort on creating policy based configuration of libvirt
objects called libvirt-designer. This is closely related to points 2)
and 3). Unfortunately there was no much going on lately and part of
virt-manager repository has currently more features implemented with
the same ideas in mind, just not exported for public use.

We could utilize some of the above to various extents.

Let me know what you think and have a nice day.
Martin

[Daniel P. Berrangé]

On Tue, Mar 20, 2018 at 03:20:31PM +0100, Martin Kletzander wrote:
> 1) Default devices/values
>
> Libvirt itself must default to whatever values there were before any
> particular element was introduced due to the fact that it strives to
> keep the guest ABI stable. That means, for example, that it can't just
> add -vmcoreinfo option (for KASLR support) or magically add the pvpanic
> device to all QEMU machines, even though it would be useful, as that
> would change the guest ABI.
>
> For default values this is even more obvious. Let's say someone figures
> out some "pretty good" default values for various HyperV enlightenment
> feature tunables. Libvirt can't magically change them, but each one of
> the projects building on top of it doesn't want to keep that list
> updated and take care of setting them in every new XML. Some projects
> don't even expose those to the end user as a knob, while others might.

This gets very tricky, very fast.

Lets say that you have an initial good set of hyperv config
tunables. Now sometime passes and it is decided that there is a
different, better set of config tunables. If the module that is
providing this policy to apps like OpenStack just updates itself
to provide this new policy, this can cause problems with the
existing deployed applications in a number of ways.

First the new config probably depends on specific versions of
libvirt and QEMU, and you can't mandate to consuming apps which
versions they must be using. So you need a matrix of libvirt +
QEMU + config option settings.

Even if you have the matching libvirt & QEMU versions, it is not
safe to assume the application will want to use the new policy.
An application may need live migration compatibility with older
versions. Or it may need to retain guaranteed ABI compatibility
with the way the VM was previously launched and be using transient
guests, generating the XML fresh each time.

The application will have knowledge about when it wants to use new
vs old hyperv tunable policy, but exposing that to your policy module
is very tricky because it is inherantly application specific logic
largely determined by the way the application code is written.

> One more thing could be automatically figuring out best values based on
> libosinfo-provided data.
>
> 2) Policies
>
> Lot of the time there are parts of the domain definition that need to be
> added, but nobody really cares about them. Sometimes it's enough to
> have few templates, another time you might want to have a policy
> per-scenario and want to combine them in various ways. For example with
> the data provided by point 1).
>
> For example if you want PCI-Express, you need the q35 machine type, but
> you don't really want to care about the machine type. Or you want to
> use SPICE, but you don't want to care about adding QXL.
>
> What if some of these policies could be specified once (using some DSL
> for example), and used by virtuned to merge them in a unified and
> predictable way?
>
> 3) Abstracting the XML
>
> This is probably just usable for stateless apps, but it might happen
> that some apps don't really want to care about the XML at all. They
> just want an abstract view of the domain, possibly add/remove a device
> and that's it. We could do that as well. I can't really tell how much
> of a demand there is for it, though.

It is safe to say that applications do not want to touch XML at all.
Any non-trivial application has created an abstraction around XML,
so that they have an API to express what they want, rather than
manipulating of strings to format/parse XML.

The libvirt-gconfig project aims to provide a C API for manipulating
XML documents, with language bindings available via GObject
introspection so you can use it from Vala, Perl, Python, JavaScript,
etc. Go is notable missing, but for that we have libvirt-go-xml
which provides a set of native Go structs to represent the XML.

The problem we've faced with libvirt-gconfig is that it is a really
hard sell to get applications to convert existing code to use it.
We've only had success where an applicaiton has been written to use
libvirt-gconfig from day one - eg GNOME Boxes and libvirt-sandbox.

Virt-manager is the poster-child for using libvirt-gconfig, but I
don't see it adopting it any time soon, as it is a massive effort
to change all existing code - even if libvirt-gconfig had full
XML schema coverge, which it doesn't :-(

I also wanted to use libvirt-gconfig in OpenStack, but there was
resistance for adding a dependancy on another native library, so
there we've basically copied what virt-manager does and defined a
set of pure python objects to represent config which is serialized
to / from XML :-( If libvirt-gconfig were to be used by OpenStack
it would need to deal with fact that not every distro has that
available, so the existing pure python config objects would need
to be maintained in parallel for an indefinite amount of time.
So in fact using libvirt-gconfig would increase maint burden for
OpenStack, rather than reduce it.

libvirt-gconfig would be a hard sell for Go apps when you compare
it to libvirt-go-xml, because the latter is following the common
Go paradigm for XML manipulation.

If there was something higher level that gets more interesting,
but the hard bit is that you still need a way to get at all the
low level bits becuase a higher level abstracted API will never
cover every niche use case.

> 4) Identifying devices properly
>
> In contrast to the previous point, stateful apps might have a problem
> identifying devices after hotplug. For example, let's say you don't
> care about the addresses and leave that up to libvirt. You hotplug a
> device into the domain and dump the new XML of it. Depending on what
> type of device it was, you might need to identify it based on different
> values. It could be <target dev=''/> for disks, <mac address=''/> for
> interfaces etc. For some devices it might not even be possible and you
> need to remember the addresses of all the previous devices and then
> parse them just to identify that one device and then throw them away.
>
> With new enough libvirt you could use the user aliases for that, but
> turns out it's not that easy to use them properly anyway. Also the
> aliases won't help users identify that device inside the guest.

NB, relating between host device config and guest visible device
config is a massive problem space in its own right, and not very
easy to address. In OpenStack we ended up defining a concept of
"device tagging" via cloud-init metadata, where openstack allows
users to set opaque string tags against devices their VM has.
OpenStack that generates a metadata file that records various
pieces of identifying hardware attributes (PCI address, MAC
addr, disk serial, etc) alongside the user tag. This metadata
file is exposed to the guest with the hope that there's enough
info to allow the user to decide which device is to be used for
which purpose

https://specs.openstack.org/openstack/nova-specs/specs/mitaka/approved/virt-device-role-tagging.html
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/10/html/networking_guide/use-tagging

> <rant>
> We really should've gone with new attribute for the user alias instead
> of using an existing one, given how many problems that is causing.
> </rant>
>
> 5) Generating the right XML snippet for device hot-(un)plug
>
> This is kind of related to some previous points.
>
> When hot-plugging a device and creating an XML snippet for it, you want
> to keep the defaults from point 1) and policies from 2) in mind. Or
> something related to the already existing domain which you can describe
> systematically. And adding something for identification (see previous
> point).
>
> Doing the hot-unplug is easy depending on how much information about
> that device is saved by your application. The less you save about the
> device (or show to the user in a GUI, if applicable) the harder it might
> be to generate an XML that libvirt will accept. Again, some problems
> with this should be fixed in libvirt, some of them are easy to
> workaround. But having a common ground that takes care of this should
> help some projects.
>
> Hot-unplug could be implemented just based on the alias. This is
> something that would fit into libvirt as well.
>
> ========================================================================
>
> To mention some pre-existing solutions:
>
> - I understand OpenStack has some really sensible and wisely chosen
> and/or tested default values.

In terms of default devices and OS specific choices, OpenStack's
decisions have been largely inspired by previous work in oVirt
and / or virt-manager. So there's obviously overlap in the
conceptual area, but there's also plenty that is very specific
to OpenStack - untangling the two extract the common bits from
the app specific bits is hard.

> - I know KubeVirt has VirtualMachinePresets. That is something closely
> related to points 1) and 2). Also their abstraction of the XML might
> be usable for point 3).
>
> - There was an effort on creating policy based configuration of libvirt
> objects called libvirt-designer. This is closely related to points 2)
> and 3). Unfortunately there was no much going on lately and part of
> virt-manager repository has currently more features implemented with
> the same ideas in mind, just not exported for public use.

This is the same kind of problem we faced wrt libvirt-gconfig and
libvirt-gobject usage from virt-manager - it has an extensive code
base that already works, and rewriting it to use something new
is alot of work for no short-term benefit. libvirt-gconfig/gobject
were supposed to be the "easy" bits for virt-manager to adopt, as
they don't really include much logic that would step on virt-manager's
toes. libvirt-designer was going to be a very opinionated library
and in retrospective that makes it even harder to consider adopting
it for usage in virt-manager, as it'll have signficant liklihood
of making functionally significant changes in behaviour.

There's also the problem with use of native libraries that would
impact many apps. We only got OpenStack to grudgingly allow the
use of libosinfo native library via GObject Introspection, by
promising to do work to turn the osinfo database into an approved
stable format which OpenStack could then consume directly, dropping
the native API usage :-( Incidentally, the former was done (formal
spec for the DB format), but the latter was not yet (direct DB usage
by OpenStack)


BTW, I don't like that I'm being so negative to your proposal :-(
I used to hope that we would be able to build higher level APIs on
top of libvirt to reduce the overlap between different applications
reinventing the wheel. Even the simplest bits we tried like the
gconfig/gobject API are barely used. libvirt-designer is basically
a failure. Though admittedly it didn't have enough development resource
applied to make it compelling, in retrospect adoption was always going
to be a hard sell except in greenfield developments.

Libosinfo is probably the bit we've had most success with, and has
most promise for the future, particularly now that we formally allow
apps to read the osinfo database directly and bypass the API. It is
quite easy to fit into existing application codebases which helps alot.
Even there I'm still disappointed that we only have GNOME Boxes using
the kickstart generator part of osinfo - oVirt and Oz both still have
their own kickstart generator code for automating OS installs.

In general though, I fear anything API based is going to be a really
hard sell to get wide adoption for based on what we've seen before.

I think the biggest bang-for-buck is identifying more areas where we
can turn code into data. There's definitely scope for recording more
types of information in the osinfo database. There might also be
scope for defining entirely new databases to complement the osinfo
data, if something looks out of scope for libosinfo.

Regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

[Eduardo Habkost]

On Tue, Mar 20, 2018 at 03:10:12PM +0000, Daniel P. Berrangé wrote:
> On Tue, Mar 20, 2018 at 03:20:31PM +0100, Martin Kletzander wrote:
> > 1) Default devices/values
> >
> > Libvirt itself must default to whatever values there were before any
> > particular element was introduced due to the fact that it strives to
> > keep the guest ABI stable. That means, for example, that it can't just
> > add -vmcoreinfo option (for KASLR support) or magically add the pvpanic
> > device to all QEMU machines, even though it would be useful, as that
> > would change the guest ABI.
> >
> > For default values this is even more obvious. Let's say someone figures
> > out some "pretty good" default values for various HyperV enlightenment
> > feature tunables. Libvirt can't magically change them, but each one of
> > the projects building on top of it doesn't want to keep that list
> > updated and take care of setting them in every new XML. Some projects
> > don't even expose those to the end user as a knob, while others might.
>
> This gets very tricky, very fast.
>
> Lets say that you have an initial good set of hyperv config
> tunables. Now sometime passes and it is decided that there is a
> different, better set of config tunables. If the module that is
> providing this policy to apps like OpenStack just updates itself
> to provide this new policy, this can cause problems with the
> existing deployed applications in a number of ways.
>
> First the new config probably depends on specific versions of
> libvirt and QEMU, and you can't mandate to consuming apps which

> versions they must be using. [...]

This is true.

> [...] So you need a matrix of libvirt +

> QEMU + config option settings.

But this is not. If config options need support on the lower
levels of the stack (libvirt and/or QEMU and/or KVM and/or host
hardware), it already has to be represented by libvirt host
capabilities somehow, so management layers know it's available.

This means any new config generation system can (and must) use
host(s) capabilities as input before generating the
configuration.

>
> Even if you have the matching libvirt & QEMU versions, it is not
> safe to assume the application will want to use the new policy.
> An application may need live migration compatibility with older
> versions. Or it may need to retain guaranteed ABI compatibility
> with the way the VM was previously launched and be using transient
> guests, generating the XML fresh each time.

Why is that a problem? If you want live migration or ABI
guarantees, you simply don't use this system to generate a new
configuration. The same way you don't use the "pc" machine-type
if you want to ensure compatibility with existing VMs.

>
> The application will have knowledge about when it wants to use new
> vs old hyperv tunable policy, but exposing that to your policy module
> is very tricky because it is inherantly application specific logic
> largely determined by the way the application code is written.

We have a huge set of features where this is simply not a
problem. For most virtual hardware features, enabling them is
not even a policy decision: it's just about telling the guest
that the feature is now available. QEMU have been enabling new
features in the "pc" machine-type for years.

Now, why can't higher layers in the stack do something similar?

The proposal is equivalent to what already happens when people
use the "pc" machine-type in their configurations, but:
1) the new defaults/features wouldn't be hidden behind a opaque
machine-type name, and would appear in the domain XML
explicitly;
2) the higher layers won't depend on QEMU introducing a new
machine-type just to have new features enabled by default;
3) features that depend on host capabilities but are available on
all hosts in a cluster can now be enabled automatically if
desired (which is something QEMU can't do because it doesn't
have enough information about the other hosts).

Choosing reasonable defaults might not be a trivial problem, but
the current approach of pushing the responsibility to management
layers doesn't improve the situation.


[...]
> > 2) Policies
[...]
> > 3) Abstracting the XML
[...]
> > 4) Identifying devices properly
[...]

> > 5) Generating the right XML snippet for device hot-(un)plug

[...]

These parts are trickier and I need to read the discussion more
carefully before replying.

--
Eduardo

[Daniel P. Berrangé]

I don't think it is that simple. The capabilities reflect what the
current host is capable of only, not whether it is desirable to
actually use them. Just because a host reports that it has q35-2.11.0
machine type doesn't mean that it should be used. The mgmt app may
only wish to use that if it is available on all hosts in a particular
grouping. The config generation library can't query every host directly
to determine this. The mgmt app may have a way to collate capabilities
info from hosts, but it is probably then stored in a app specific
format and data source, or it may just ends up being a global config
parameter to the mgmt app per host.

There have been a number of times where a feature is available in
libvirt and/or QEMU, and the mgmt app still doesn't yet may still
not wish to use it because it is known broken / incompatible with
certain usage patterns. So the mgmt app would require an arbitrarily
newer libvirt/qemu before considering using it, regardless of
whether host capabilities report it is available.

> > Even if you have the matching libvirt & QEMU versions, it is not
> > safe to assume the application will want to use the new policy.
> > An application may need live migration compatibility with older
> > versions. Or it may need to retain guaranteed ABI compatibility
> > with the way the VM was previously launched and be using transient
> > guests, generating the XML fresh each time.
>
> Why is that a problem? If you want live migration or ABI
> guarantees, you simply don't use this system to generate a new
> configuration. The same way you don't use the "pc" machine-type
> if you want to ensure compatibility with existing VMs.

In many mgmt apps, every VM potentially needs live migration, so
unless I'm misunderstanding, you're effectively saying don't ever
use this config generator in these apps.

> > The application will have knowledge about when it wants to use new
> > vs old hyperv tunable policy, but exposing that to your policy module
> > is very tricky because it is inherantly application specific logic
> > largely determined by the way the application code is written.
>
> We have a huge set of features where this is simply not a
> problem. For most virtual hardware features, enabling them is
> not even a policy decision: it's just about telling the guest
> that the feature is now available. QEMU have been enabling new
> features in the "pc" machine-type for years.
>
> Now, why can't higher layers in the stack do something similar?
>
> The proposal is equivalent to what already happens when people
> use the "pc" machine-type in their configurations, but:
> 1) the new defaults/features wouldn't be hidden behind a opaque
> machine-type name, and would appear in the domain XML
> explicitly;
> 2) the higher layers won't depend on QEMU introducing a new
> machine-type just to have new features enabled by default;
> 3) features that depend on host capabilities but are available on
> all hosts in a cluster can now be enabled automatically if
> desired (which is something QEMU can't do because it doesn't
> have enough information about the other hosts).
>
> Choosing reasonable defaults might not be a trivial problem, but
> the current approach of pushing the responsibility to management
> layers doesn't improve the situation.

The simple cases have been added to the "pc" machine type, but
more complex cases have not been dealt with as they often require
contextual knowledge of either the host setup or the guest OS
choice.

We had a long debate over the best aio=threads,native setting for
OpenStack. Understanding the right defaults required knowledge about
the various different ways that Nova would setup its storage stack.
We certainly know enough now to be able to provide good recommendations
for the choice, with perf data to back it up, but interpreting those
recommendations still requires the app specific knowledge about its
storage mgmt approach, so ends up being code dev work.

Another case is the pvpanic device - while in theory that could
have been enabled by default for all guests, by QEMU or a config
generator library, doing so is not useful on its own. The hard
bit of the work is adding code to the mgmt app to choose the
action for when pvpanic triggers, and code to handle the results
of that action.

[Eduardo Habkost]

In other words, you need host capabilities from all hosts as
input when generating a new config XML. We already have a format
to represent host capabilities defined by libvirt, users of the
new system would just need to reproduce the data they got from
libvirt and give it to the config generator.

Not completely trivial, but maybe worth the effort if you want to
benefit from work done by other people to find good defaults?

>
> There have been a number of times where a feature is available in
> libvirt and/or QEMU, and the mgmt app still doesn't yet may still
> not wish to use it because it is known broken / incompatible with
> certain usage patterns. So the mgmt app would require an arbitrarily
> newer libvirt/qemu before considering using it, regardless of
> whether host capabilities report it is available.

If this happens sometimes, why is it better for the teams
maintaining management layers to duplicate the work of finding
what works, instead of solving the problem only once?

>
> > > Even if you have the matching libvirt & QEMU versions, it is not
> > > safe to assume the application will want to use the new policy.
> > > An application may need live migration compatibility with older
> > > versions. Or it may need to retain guaranteed ABI compatibility
> > > with the way the VM was previously launched and be using transient
> > > guests, generating the XML fresh each time.
> >
> > Why is that a problem? If you want live migration or ABI
> > guarantees, you simply don't use this system to generate a new
> > configuration. The same way you don't use the "pc" machine-type
> > if you want to ensure compatibility with existing VMs.
>
> In many mgmt apps, every VM potentially needs live migration, so
> unless I'm misunderstanding, you're effectively saying don't ever
> use this config generator in these apps.

If you only need live migration, you can choose between:
a) not using it;
b) using an empty host capability list as input when generating the
XML (maybe this would be completely useless, but it's still an
option);
c) use only host _software_ capabilities as input, if you control
the software that runs on all hosts.
d) use an intersection of the software+host capabilities of all
hosts as input.

If you care about 100% static guest ABI (not just live
migration), you either generate the XML once and save it for
later, or you don't use the config generation system. (IOW, the
same limitations as the "pc" machine-type alias).

Exactly. But on how many of those cases the decision requires
knowledge that is specific to the management stack being used
(like the ones you listed below), and how many are decisions that
could be made by simply looking at the host software/hardware and
guest OS? I am under the impression that we have a reasonable
number of case of the latter.

The ones I remember are all relate to CPU configuration:
* Automatically enabling useful CPU features when they are
available on all hosts;
* Always enabling check='full' by default.

Do we have other examples?

>
> We had a long debate over the best aio=threads,native setting for
> OpenStack. Understanding the right defaults required knowledge about
> the various different ways that Nova would setup its storage stack.
> We certainly know enough now to be able to provide good recommendations
> for the choice, with perf data to back it up, but interpreting those
> recommendations still requires the app specific knowledge about its
> storage mgmt approach, so ends up being code dev work.
>
> Another case is the pvpanic device - while in theory that could
> have been enabled by default for all guests, by QEMU or a config
> generator library, doing so is not useful on its own. The hard
> bit of the work is adding code to the mgmt app to choose the
> action for when pvpanic triggers, and code to handle the results
> of that action.
>
>
> Regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

--
Eduardo

[Daniel P. Berrangé]

On Thu, Mar 22, 2018 at 09:40:40AM +0100, Pavel Hrdina wrote:
> On Tue, Mar 20, 2018 at 03:10:12PM +0000, Daniel P. Berrangé wrote:

> > > - I understand OpenStack has some really sensible and wisely chosen
> > > and/or tested default values.
> >
> > In terms of default devices and OS specific choices, OpenStack's
> > decisions have been largely inspired by previous work in oVirt
> > and / or virt-manager. So there's obviously overlap in the
> > conceptual area, but there's also plenty that is very specific
> > to OpenStack - untangling the two extract the common bits from
> > the app specific bits is hard.
>

> This would be handled by the application specific policies. The
> virtuned will have some reasonable defaults that are known to work in
> most cases and suits the majority of users, but it's clear that
> sometimes you need some specific defaults and you would provide them
> via the application policy.
>
> For example, to create a XML for windows guest the virtuned would not
> probably select virtio devices because there are no drivers for them
> in the standard windows installation, however, some management
> application may have customized preinstalled disk images or customized
> ISO images or it may be able to provide the drivers any other way, so
> they would specify in the application policy that for windows guest
> virtuned should use virtio as a default device model.

As soon as we talk about configuring hardware specific to a guest
OS, then that is in scope of existing libosinfo project, not something
we should create a new project for.


> This is probably the hardest part of creating higher level API on top
> of libvirt, not every project may be willing to rewrite their existing
> code. On the other hand, I know that for example Cockpit would benefit
> from the virtuned providing this functionality via REST API.
>
> It's a chicken and egg problem, but if we can gather input from all the
> existing projects that have their own implementation and figure out how
> to make virtuned usable for all of them they might consider to start
> using it.

I don't doubt that Cockpit would like like, but based on previous efforts
we've made I'm sceptical that anything beyond Cockpit would use any new
API.

[Daniel P. Berrangé]

Things aren't that simple - when openstack reports info from each host
it doesn't do it in any libvirt format - it uses an arbitrary format it
defines itself. Going from libvirt host capabilities to the app specific
format and back to libvirt host capabilities will loose information.
Then you also have matter of hosts coming & going over time, so fragile
to assume that the set of host capabilities you currently see are
representative of the steady state you desire.

> Not completely trivial, but maybe worth the effort if you want to
> benefit from work done by other people to find good defaults?

Perhaps, but there's many ways to share the work of figuring out
good defaults. Beyond what's represented in libosinfo database,
no one has even tried to document what current desirable defaults
are. Jumping straight from no documented best practice, to lets
build a API is a big ask, particularly when the suggestion involves
major architectural changes to any app that wants to use it.

For most immediate benefit actually documenting some best practice
would be the most tangible win for application developers, as they
can much more easily adapt existing code to follow it. ALso expanding
range of info we record in libosinfo would be beneficial, since there
is still plenty of OS specific data not captured. Not to mention that
most applications aren't even leveraging much of the stuff already
available.

> > There have been a number of times where a feature is available in
> > libvirt and/or QEMU, and the mgmt app still doesn't yet may still
> > not wish to use it because it is known broken / incompatible with
> > certain usage patterns. So the mgmt app would require an arbitrarily
> > newer libvirt/qemu before considering using it, regardless of
> > whether host capabilities report it is available.
>
> If this happens sometimes, why is it better for the teams
> maintaining management layers to duplicate the work of finding
> what works, instead of solving the problem only once?

This point was in relation to my earlier thread where I said that
it would be neccessary to maintain a matrix of policy vs QEMU and
libvirt versions, not merely relying on host capabilities.

Anything todo with virtual hardware that is guest OS dependant
should be in scope of libosinfo project / database.

For other things, I think it would be useful if we at least started
to document some recommended best practices, so we have a better idea
of what we're trying to address. It would also give apps an idea of
what they're missing right now letting them fix gaps, if desired.

> The ones I remember are all relate to CPU configuration:
> * Automatically enabling useful CPU features when they are
> available on all hosts;

This is really hard todo in an automated fashion because it
relies on having an accessible global view of all hosts, that is
accurate. I can easily see a situation where you have 20 hosts, 5
old CPUs, 15 new CPUs, and the old ones are coincidentally offline
for maintenance or software upgrade. Meanwhile you spawn a guest,
and check available host capabilities and never see the info from
older CPUs, so automatically enable a bunch of features that we
really did not want. It is more reliable if you just declare this
in the application config file, and have a mgmt tool that can
do distributed updates of the config file when needed.

> * Always enabling check='full' by default.
>
> Do we have other examples?

I'm sure we can find plenty, but its a matter of someone doing the
work to investigate & pull together docs.

[Michal Skrivanek]

We have an opportunity to make kubevirt right, too. Sure, the project is halfway through with own presets and custom code (just repeating oVirt and Openstack), but it’s not too late to change things there, and a remote API fits well

>
> Regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
>

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[Eduardo Habkost]

Well, then the management layer should stop losing useful data. ;)

(But I understand that this is not that simple)

>
> > Not completely trivial, but maybe worth the effort if you want to
> > benefit from work done by other people to find good defaults?
>
> Perhaps, but there's many ways to share the work of figuring out
> good defaults. Beyond what's represented in libosinfo database,
> no one has even tried to document what current desirable defaults
> are. Jumping straight from no documented best practice, to lets
> build a API is a big ask, particularly when the suggestion involves
> major architectural changes to any app that wants to use it.
>
> For most immediate benefit actually documenting some best practice
> would be the most tangible win for application developers, as they
> can much more easily adapt existing code to follow it. ALso expanding
> range of info we record in libosinfo would be beneficial, since there
> is still plenty of OS specific data not captured. Not to mention that
> most applications aren't even leveraging much of the stuff already
> available.
>

This is a good point.

>
> > > There have been a number of times where a feature is available in
> > > libvirt and/or QEMU, and the mgmt app still doesn't yet may still
> > > not wish to use it because it is known broken / incompatible with
> > > certain usage patterns. So the mgmt app would require an arbitrarily
> > > newer libvirt/qemu before considering using it, regardless of
> > > whether host capabilities report it is available.
> >
> > If this happens sometimes, why is it better for the teams
> > maintaining management layers to duplicate the work of finding
> > what works, instead of solving the problem only once?
>
> This point was in relation to my earlier thread where I said that
> it would be neccessary to maintain a matrix of policy vs QEMU and
> libvirt versions, not merely relying on host capabilities.

I see what you mean. But any component in the system needs to
keep a matrix of QEMU and libvirt versions, I'd argue that the
APIs provided by QEMU & libvirt are broken and need to be fixed.

If this happens with QEMU, I ask for everybody involved to please
ask QEMU developers for help, so we can at least document the
issue, and find a better way to detect if a given feature is
working.

(This request applies even if our effort is focused towards
documenting best practices and not an API.)

Agreed, though I expect any documented best practices will also
end up including guest-specific recommendations (which may or may
not be already in the libosinfo database).

>
> > The ones I remember are all relate to CPU configuration:
> > * Automatically enabling useful CPU features when they are
> > available on all hosts;
>
> This is really hard todo in an automated fashion because it
> relies on having an accessible global view of all hosts, that is
> accurate. I can easily see a situation where you have 20 hosts, 5
> old CPUs, 15 new CPUs, and the old ones are coincidentally offline
> for maintenance or software upgrade. Meanwhile you spawn a guest,
> and check available host capabilities and never see the info from
> older CPUs, so automatically enable a bunch of features that we
> really did not want. It is more reliable if you just declare this
> in the application config file, and have a mgmt tool that can
> do distributed updates of the config file when needed.

This surprises me a bit. I really expect any management layer to
have an accurate and updated global view of all hosts, even if
they are temporarily offline. And if new hosts are expected to
be added in the future, the system should have a very clearly
defined expectation[1] of what are the minimal capabilities
required for new hosts.

Otherwise we will be pushing complex decisions to human operators
(which will probably end up making worse mistakes because they
may not understand how everything works internally).

>
> > * Always enabling check='full' by default.
> >
> > Do we have other examples?
>
> I'm sure we can find plenty, but its a matter of someone doing the
> work to investigate & pull together docs.

Agreed that documenting this stuff is the most important step
right now.

>
> > > We had a long debate over the best aio=threads,native setting for
> > > OpenStack. Understanding the right defaults required knowledge about
> > > the various different ways that Nova would setup its storage stack.
> > > We certainly know enough now to be able to provide good recommendations
> > > for the choice, with perf data to back it up, but interpreting those
> > > recommendations still requires the app specific knowledge about its
> > > storage mgmt approach, so ends up being code dev work.
> > >
> > > Another case is the pvpanic device - while in theory that could
> > > have been enabled by default for all guests, by QEMU or a config
> > > generator library, doing so is not useful on its own. The hard
> > > bit of the work is adding code to the mgmt app to choose the
> > > action for when pvpanic triggers, and code to handle the results
> > > of that action.
>
> Regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

--
Eduardo

[Martin Kletzander]

[ I fixed up ovirt...@redhat.com to be de...@ovirt.org since the
former is deprecated. I'm also not trimming down much of the reply so
that they can get the whole picture. Sorry for the confusion ]

On Tue, Mar 20, 2018 at 03:10:12PM +0000, Daniel P. Berrangé wrote:

The idea was for updating XML based on policy, which is something you
want for new machines. You should then keep the XML per domain and only
do changes to if requested by the user or when libvirt fills in new
values in a guest ABI compatible fashion.

>
>> One more thing could be automatically figuring out best values based on
>> libosinfo-provided data.
>>
>> 2) Policies
>>
>> Lot of the time there are parts of the domain definition that need to be
>> added, but nobody really cares about them. Sometimes it's enough to
>> have few templates, another time you might want to have a policy
>> per-scenario and want to combine them in various ways. For example with
>> the data provided by point 1).
>>
>> For example if you want PCI-Express, you need the q35 machine type, but
>> you don't really want to care about the machine type. Or you want to
>> use SPICE, but you don't want to care about adding QXL.
>>
>> What if some of these policies could be specified once (using some DSL
>> for example), and used by virtuned to merge them in a unified and
>> predictable way?
>>
>> 3) Abstracting the XML
>>
>> This is probably just usable for stateless apps, but it might happen
>> that some apps don't really want to care about the XML at all. They
>> just want an abstract view of the domain, possibly add/remove a device
>> and that's it. We could do that as well. I can't really tell how much
>> of a demand there is for it, though.
>
>It is safe to say that applications do not want to touch XML at all.
>Any non-trivial application has created an abstraction around XML,
>so that they have an API to express what they want, rather than
>manipulating of strings to format/parse XML.
>

Sure, this was just meant to be a question as to whether it's worth
pursuing or not. You make a good point on why it is not (at least for
existing apps).

However, since this was optional, the way this would look without the
XML abstraction is that both input and output would be valid domain
definitions, ultimately resulting in something similar to virt-xml with
the added benefit of applying a policy from a file/string either
supplied by the application itself. Whether that policy was taken from
a common repository of such knowledge is orthogonal to this idea. Since
you would work with the same data, the upgrade could be incremental as
you'd only let virtuned fill in values for new options and could slowly
move on to using it for some pre-existing ones. None of the previous
approaches did this, if I'm not mistaken. Of course it gets more
difficult when you need to expose all the bits libvirt does and keep
them in sync (as you write below).

[...]

>If there was something higher level that gets more interesting,
>but the hard bit is that you still need a way to get at all the
>low level bits becuase a higher level abstracted API will never
>cover every niche use case.
>

Oh, definitely not every, but I see two groups of projects that have a
lot in common between themselves and between the groups as well. On the
other hand just templating and defaults is something that's easy enough
to do that it's not worth outsourcing that into another one's codebase.

>> 4) Identifying devices properly
>>
>> In contrast to the previous point, stateful apps might have a problem
>> identifying devices after hotplug. For example, let's say you don't
>> care about the addresses and leave that up to libvirt. You hotplug a
>> device into the domain and dump the new XML of it. Depending on what
>> type of device it was, you might need to identify it based on different
>> values. It could be <target dev=''/> for disks, <mac address=''/> for
>> interfaces etc. For some devices it might not even be possible and you
>> need to remember the addresses of all the previous devices and then
>> parse them just to identify that one device and then throw them away.
>>
>> With new enough libvirt you could use the user aliases for that, but
>> turns out it's not that easy to use them properly anyway. Also the
>> aliases won't help users identify that device inside the guest.
>
>NB, relating between host device config and guest visible device
>config is a massive problem space in its own right, and not very
>easy to address. In OpenStack we ended up defining a concept of
>"device tagging" via cloud-init metadata, where openstack allows
>users to set opaque string tags against devices their VM has.
>OpenStack that generates a metadata file that records various
>pieces of identifying hardware attributes (PCI address, MAC
>addr, disk serial, etc) alongside the user tag. This metadata
>file is exposed to the guest with the hope that there's enough
>info to allow the user to decide which device is to be used for
>which purpose
>

This is good point, but I was mostly thinking about identifying devices
from the host POV between two different XMLs (pre- and post- some
XML-modifying action, like hotplug).

It definitely is, but do you think it's so difficult it's worthless to
pursuit? I did a tiny PoC based on the code from virt-manager, which
was trivial mainly thanks to the XMLBuilder for the domain objects.
Maybe exposing an easy way to work with the XML would be enough for some
projects.

Little birdie from oVirt told me that they would like some of sort of
thing that does what you can achieve with virt-xml if we, for example,
made it work on pure XML definitions without connecting to libvirt.

>> - I know KubeVirt has VirtualMachinePresets. That is something closely
>> related to points 1) and 2). Also their abstraction of the XML might
>> be usable for point 3).
>>
>> - There was an effort on creating policy based configuration of libvirt
>> objects called libvirt-designer. This is closely related to points 2)
>> and 3). Unfortunately there was no much going on lately and part of
>> virt-manager repository has currently more features implemented with
>> the same ideas in mind, just not exported for public use.
>
>This is the same kind of problem we faced wrt libvirt-gconfig and
>libvirt-gobject usage from virt-manager - it has an extensive code
>base that already works, and rewriting it to use something new
>is alot of work for no short-term benefit. libvirt-gconfig/gobject
>were supposed to be the "easy" bits for virt-manager to adopt, as
>they don't really include much logic that would step on virt-manager's
>toes. libvirt-designer was going to be a very opinionated library
>and in retrospective that makes it even harder to consider adopting
>it for usage in virt-manager, as it'll have signficant liklihood
>of making functionally significant changes in behaviour.
>

The initial idea (which I forgot to mention) was that all the decisions
libvirt currently does (so that it keeps the guest ABI stable) would be
moved into data (let's say some DSL) and it could then be switched or
adjusted if that's not what the mgmt app wants (on a per-definition
basis, of course). I didn't feel very optimistic about the upstream
acceptance for that idea, so I figured that there could be something
that lives beside libvirt, helps with some policies if requested and
then the resulting XML could be fed into libvirt for determining the
rest.

>There's also the problem with use of native libraries that would
>impact many apps. We only got OpenStack to grudgingly allow the

By native you mean actual binary libraries or native to the OpenStack
code as in python module? Because what I had in mind for this project
was a python module with optional wrapper for REST API.

>use of libosinfo native library via GObject Introspection, by
>promising to do work to turn the osinfo database into an approved
>stable format which OpenStack could then consume directly, dropping
>the native API usage :-( Incidentally, the former was done (formal
>spec for the DB format), but the latter was not yet (direct DB usage
>by OpenStack)
>
>
>BTW, I don't like that I'm being so negative to your proposal :-(
>I used to hope that we would be able to build higher level APIs on
>top of libvirt to reduce the overlap between different applications
>reinventing the wheel. Even the simplest bits we tried like the
>gconfig/gobject API are barely used. libvirt-designer is basically
>a failure. Though admittedly it didn't have enough development resource
>applied to make it compelling, in retrospect adoption was always going
>to be a hard sell except in greenfield developments.
>

I'm glad for the knowledge you provided. So maybe instead of focusing
on de-duplication of existing codebases we could _at least_ aim at
future mgmt apps. OTOH improving documentation on how to properly build
higher level concepts on top of libvirt would benefit them as well.

[Daniel P. Berrangé]

That has implications for how mgmt app deals with XML. Nova has object
models for representing XML in memory, but it doesn't aim to have
loss-less roundtrip from parse -> object -> format. So if Nova gets
basic XML from virttuned, parses it into its object to let it set
more fields and then formats it again, chances are it will have lost
a bunch of stuff from virttuned. Of course if you know about this
need upfront you can design the application such that it can safely
round-trip, but this is just example of problem with integrating to
existing apps.

The other thing that concerns is that there are dependancies between
different bits of XML for a given device. ie if feature X is set to
a certain value, that prevents use of feature Y. So if virttuned
sets feature X, but the downstream application uses feature Y, the
final result can be incompatible. The application won't know this
because it doesn't control what stuff virttuned would be setting.
This can in turn cause ordering constraints.

eg the application needs to say that virtio-net is being used, then
virttuned can set some defaults like enabling vhost-net, and then
the application can fill in more bits that it cares about. Or if
we let virttuned go first, setting virtio-net model + vhost-net,
then application wants to change model to e1000e, it has to be
aware that it must now delete the vhost-net bit that virtuned
added. This ends up being more complicated that just ignoring
virttuned and coding up use of vhost-net in application code.

> > This is the same kind of problem we faced wrt libvirt-gconfig and
> > libvirt-gobject usage from virt-manager - it has an extensive code
> > base that already works, and rewriting it to use something new
> > is alot of work for no short-term benefit. libvirt-gconfig/gobject
> > were supposed to be the "easy" bits for virt-manager to adopt, as
> > they don't really include much logic that would step on virt-manager's
> > toes. libvirt-designer was going to be a very opinionated library
> > and in retrospective that makes it even harder to consider adopting
> > it for usage in virt-manager, as it'll have signficant liklihood
> > of making functionally significant changes in behaviour.
> >
>
> The initial idea (which I forgot to mention) was that all the decisions
> libvirt currently does (so that it keeps the guest ABI stable) would be
> moved into data (let's say some DSL) and it could then be switched or
> adjusted if that's not what the mgmt app wants (on a per-definition
> basis, of course). I didn't feel very optimistic about the upstream
> acceptance for that idea, so I figured that there could be something
> that lives beside libvirt, helps with some policies if requested and
> then the resulting XML could be fed into libvirt for determining the
> rest.

I can't even imagine how we would go about encoding the stable guest
ABI logic libvirt does today in data !

>
> > There's also the problem with use of native libraries that would
> > impact many apps. We only got OpenStack to grudgingly allow the
>
> By native you mean actual binary libraries or native to the OpenStack
> code as in python module? Because what I had in mind for this project
> was a python module with optional wrapper for REST API.

I meant native binary libraries. ie openstack is not happy in general
with adding dependancies on new OS services, because there's a big
time lag for getting them into all distros. By comparison a pure
python library, they can just handle automatically in their deployment
tools, just pip installing on any OS distro straight from pypi. This
is what made use of libosinfo a hard sell in Nova.

The same thing is seen with Go / Rust where some applications have
decided they're better of actually re-implementing the libvirt RPC
protocol in Go / Rust rather than use the libvirt.so client. I think
this is a bad tradeoff in general, but I can see why they like it

[Yaniv Lavi]

[resending to include KubeVirt devs ]

YANIV LAVI
SENIOR TECHNICAL PRODUCT MANAGER
Red Hat Israel Ltd.
34 Jerusalem Road, Building A, 1st floor
Ra'anana, Israel 4350109
yl...@redhat.com    T: +972-9-7692306/8272306     F: +972-9-7692223    IM: ylavi
TRIED. TESTED. TRUSTED.
@redhatnews   Red Hat   Red Hat

On Wed, Apr 4, 2018 at 7:07 PM, Yaniv Lavi <yl...@redhat.com> wrote:

Hi,

I'd like to go one step back and discuss why we should try to do this on the high level.

For the last 5-10 years of KVM development, we are pragmatically providing the Linux host level APIs via project specific host agents/integration code (Nova agent, oVirt host agent, virt-manager).

In recent time we see new projects that have similar requirements (Cockpit, different automation tool, KubeVirt), this means that all of the Linux virt stack consumers are reinventing the wheel and using very different paths to consume the partial solutions that are provided today.

The use of the Linux virt stack is well defined by the existing projects scope and it makes a lot of sense to try to provide the common patterns via the virt stack directly as a host level API that different client or management consume.

The main goal is to improve the developer experience for virtualization management applications with an API set that is useful to the entire set of tools (OSP, oVirt, KubeVirt, Cockpit and so on).

The Linux virt developer community currently is not able to provide best practices and optimizations from single node knowledge. This means that all of that smarts is locked to the specific project integration in the good case or not provided at all and the projects as a whole lose from that. When testing the Linux virt stack itself and since each project has different usage pattern, we lose the ability to test abilities on the lower level making the entire stack less stable and complete for new features. 

This also limits the different projects ability to contribute back to the Linux stack based on their user and market experience for others in open source to gain.

I understand this shift is technically challenging for existing projects, but I do see value in doing this even for new implementation like Cockpit and KubeVirt.

I also believe that the end result could be appealing enough to cause project like OSP, virt-manager and oVirt to consider to reduce the existing capabilities of their host side integrations/agents to shims on the host level and reuse the common/better-tested pattern as clients that was developed against the experience of the different projects.

I call us all to collaborate and try to converge on a solution that will help all in the long term in the value you get from the common base.

Thanks,

YANIV LAVI
SENIOR TECHNICAL PRODUCT MANAGER
Red Hat Israel Ltd.
34 Jerusalem Road, Building A, 1st floor
Ra'anana, Israel 4350109
yl...@redhat.com    T: +972-9-7692306/8272306     F: +972-9-7692223    IM: ylavi
TRIED. TESTED. TRUSTED.
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_______________________________________________
Devel mailing list
De...@ovirt.org
http://lists.ovirt.org/mailman/listinfo/devel

[Yaniv Lavi]

[resending to include OSP devs ]

YANIV LAVI
SENIOR TECHNICAL PRODUCT MANAGER
Red Hat Israel Ltd.
34 Jerusalem Road, Building A, 1st floor
Ra'anana, Israel 4350109
yl...@redhat.com    T: +972-9-7692306/8272306     F: +972-9-7692223    IM: ylavi
TRIED. TESTED. TRUSTED.
@redhatnews   Red Hat   Red Hat