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On Thu, Sep 15, 2022 at 01:56:06PM +0100, Julien Grall wrote:
> Hi Demi,
> This raises two questions:
> 1) What is the performance impact to turn this on by default? I am looking
> for actual numbers.
I do not have access to such hardware and so cannot provide such
numbers. I was hoping that someone else would be able to do the needed
> 2) What happen on HW that doesn't support DIT? Are we going to mark them as
The relevant text in Intel’s documentation is:
> For Intel® Core™ family processors based on microarchitectures before
> Ice Lake and Intel Atom® family processors based on microarchitectures
> before Gracemont that do not enumerate IA32_UARCH_MISC_CTL, developers
> may assume that the instructions listed here operate as if DOITM is
> Intel Core family processors based on Ice Lake and later, such as
> Tiger Lake, Lakefield, and Rocket Lake will enumerate DOITM. Intel
> Atom processors based on Gracemont and later will also enumerate
> DOITM. Refer to the Enumeration and Architectural MSRs section for
> more information.
In other words, no action is needed (or possible) on CPUs that do not
enumerate DOITM. CPUs that do enumerate DOITM require it to be
explicitly enabled by for cryptographic code to be secure. This was a
poor design decision on Intel’s part, which might be why it appears that
Linux will treat DOITM as a CPU bug.
> > Otherwise any cryptographic code in the guests thinks
> > it is constant time when it may not be.
> Why would a guest think that? Are we telling the guest DIT is supported but
> doesn't honour it?
Xen is telling guests that DOITM is not required for constant-time
operation of cryptographic code, even though the hardware actually
requires it. Furthermore, Xen does not allow guests to set DOITM.
> If yes, then I would argue that we should clear that bit. Otherwise...
Xen actually needs to *set* that bit, not clear it.
> > Once guests have the ability to
> > control this I would be open to reconsidering this.
> ... this will introduce a problem once we expose it to the guest because we
> cannot change the global default as some user my start to rely on it on the
I would be fine with requiring the toolstack to opt-out from the safe
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
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