Notes: 2026-06-29 CC Virtualization Hypervisor/Kernel subgroup

[Julianus Larson]

Hi, 

Here are the notes from yesterday's meeting. Thanks to everybody who participated. Bertrand's slides are attached.

Best regards,

Julianus

Attendance:

Mark Burton
Bertrand Marquis
Manos Pitsidianakis
Julianus Larson
Edgar Iglesias
Alex Benée 
Bill Mills 
Stefano Stabellini
Don Harbin 
Michael Peter
Sumit Semwal
Randy Linell 
Joyce QI

Quick recap
This was the second meeting of the Hypervisor Kernel subgroup of the Core collective virtualization group, focused on discussing nested virtualization and real-time support. Bertrand proposed reorganizing the virtualization groups, suggesting renaming the kernel hypervisor group to a Xen subgroup, removing the FFA subgroup, and renaming the simulation group to QEMU KVM, which received approval from Alex. The main discussion centered on nested virtualization use cases, where Stefano explained two scenarios: running existing KVM systems on top of Xen and vice versa, with particular interest from AMD and Qualcomm in simulation and acceleration work. Alex detailed technical approaches including modifying KVM to trap everything or implementing a sub-hypervisor layer, while Mark emphasized the potential performance benefits for TCG simulation. The group decided to move the nested virtualization discussion to the QEMU simulation working group, with Alex agreeing to write up a brief introduction document and lead the discussion there. The meeting also touched on real-time virtualization needs, particularly in automotive and robotic applications, though detailed use cases and implementation approaches were left for future discussion.

Next steps

Alex

• Write up a brief introduction with diagrams and details on the nested virtualization/simulation acceleration initiative for socialization and discussion in the QEMU/KVM subgroup.
• Move the nested virtualization/simulation acceleration topic to the QEMU/KVM subgroup for further discussion and collaboration.

Bertrand

• Share the meeting notes/slides with the group after the meeting.
• Ping contacts (internally and externally) about interest in nested virtualization, especially involving Google.
• Follow up internally and externally with people who might have interest in nested virtualization.

Edgar

• Discuss the nested virtualization/simulation acceleration topic with the AMD QEMU simulation team and provide feedback.

Julianus

• Schedule the next meeting for the same time in 3 weeks.

Sumit

• Check with contacts at Google about interest in the nested virtualization topic and provide a feeler.

Summary

Hypervisor Group Restructuring Proposal

Bertrand led the second meeting of the Hypervisor Kernel subgroup and proposed organizational changes for the virtualization group. He suggested renaming the kernel hypervisor group to a Xen subgroup, removing the FFA subgroup, renaming the simulation group to QEMU KVM, and eliminating the dedicated maintenance group in favor of monthly cross-sync meetings. Bertrand requested feedback from the group on these proposed changes, emphasizing that this was still a suggestion rather than a decision.

Nested Virtualization Use Cases Discussion

The group discussed nested virtualization use cases, with Stefano explaining two main scenarios: running virtualization stacks within Xen-based systems and running Xen stacks on top of KVM systems. Bertrand confirmed that KVM already supports nested virtualization, and Mark noted that while Qualcomm has some internal simulation uses, the focus should be on exit handling rather than acceleration. Stefano indicated willingness to work on nested virtualization implementation but expressed interest in collaboration with others, which aligns with the group's purpose of coordinating efforts.

Nested Virtualization Contributor Discussion

The team discussed potential contributors for nested virtualization work in Xen, with Bertrand and Sumit agreeing to reach out to contacts at Google and ARM to gauge interest. The group explored technical approaches for implementing sub-hypervisor functionality, with Alex explaining the concept of running a nested EL2 hypervisor to trap and control exceptions that KVM otherwise handles internally. While modifying KVM was considered, the team discussed the challenges of getting such patches upstreamed and explored the alternative approach of implementing a custom nested hypervisor, which would provide full control over memory mappings and trap configurations.

KVM Hypervisor Implementation Discussion

The group discussed implementing a hypervisor configuration for simulation work, with Alex explaining how KVM would be easier to implement than Xen due to KVM's per-VCPU interface. Stefano mentioned a similar x86 configuration used to translate PV guests to HVM guests and indicated interest in collaborating on interrupt latency and real-time work. Mark suggested tracking the work in the simulation working group and noted that while multiple approaches might be suggested, expert opinion should be considered with caution.

Nested Virtualization for TCG Simulation

The group discussed using nested virtualization in QEMU to potentially accelerate TCG simulation, particularly for Qualcomm's Hexagon architecture. Mark and Alex explained that while this approach could significantly speed up computation, I/O operations would still be slow due to the overhead of trapping and emulating. The team agreed to move the discussion to the QEMU simulation group for further exploration, with Alex taking the lead on presenting the proposal there and seeking input from other stakeholders like AMD.

XEN SDG Virtualization on Android

The team discussed XEN SDG virtualization support on Android, with Bertrand noting that Michael's involvement is needed before any commitments can be made. Alex agreed to create a brief write-up to help socialize the topic, as they were already preparing for an upcoming Wednesday meeting. The group then explored real-time virtualization use cases, with Bertrand identifying automotive and robotic applications as key areas, noting that robotic use cases have less safety certification constraints than automotive applications.

CrossVM on Xen Investigation

Michael explained that they are investigating ways to run CrossVM on Xen for automotive use cases, as native KVM is not preferred in this context. Two potential pathways were discussed: porting CrossVM natively to Xen or running CrossVM with KVM on top of Xen. Michael noted that they lack experience with Xen and would need more information about existing hypervisor ports to determine the feasibility of either approach. The team expressed interest in CrossVM's well-maintained VirtIO backends, which could provide hardware capabilities beneficial for automotive development.

CrossVM Nested Virtualization Discussion

The team discussed the feasibility of using CrossVM for nested virtualization as a stepping stone to achieve their goals, particularly for Android environment development. Bertrand raised concerns about the complexity of hardware access and pass-through in nested VMs, while Michael acknowledged these challenges but emphasized that CrossVM could serve as a pathway, especially for features like GPU virtualization. The group agreed to continue the discussion in three weeks, with Sumit offering to drive the next meeting's discussion, and Julianus confirming the scheduling.

[Nathalie Chan King Choy]

Hi all,

Since it is Friday, making sure everyone's reminded of Monday's upcoming Virtualization Xen call.  

Julianus is OOO and I'm covering this subgroup for him.  

I did a quick review of your 6/29 call notes (included below for quick reference) and saw that the discussion on CrossVM Nested Virtualization will continue on 7/20 with @Sumit Semwal  offering to drive the discussion.

We should also discuss if you want to keep/cancel/reschedule the 7/27 call.

@bertrand...@arm.com , please let us know if you have additional agenda topics.

Wishing you all a great weekend,

Nathalie

--

CoreCollective Community Manager 

Android & WoA working groups

[Sumit Semwal]

Hi Nathalie,

Thanks for your email.

I did get around to check with Google Android folks about the nested Virtualisation.

So far I've not had any indications about Android folks' specific interest in nested virtualisation based on my interaction with the few folks.

I don't have any other agenda for this week and I think Bertrand may also be out (I don't remember if he's back in the coming week).

Since we've not heard from any other folks about Nested Virt, I wonder if it makes sense to cancel this Monday and keep the 7/27 call? That might give us a little more space to socialize and check interest.

Thanks and regards,

Sumit Semwal (he / him)
Senior Tech Lead - Platforms and Virtualisation
Linaro.org │ Arm Solutions at Light Speed

[Nathalie Chan King Choy]

Hi Sumit,

Thanks for the update & your guidance.  I didn't get replies from Bertrand this week, so your guess of him being OOO would make sense.

I tried to cancel the Monday 7/20 call from the Virtualization Google Calendar, but it appears that it may have come from Julianus' calendar. The deletion of the instance in the  the Virtualization Google Calendar didn't affect the instance in my calendar... so I updated the subject of this email & hopefully everyone sees it.

Hi all,

If someone from Arm is able to see Bertrand's calendar, please let me know if he'll be back for  the 7/27 call.

Thanks & regards,

Nathalie

--

CoreCollective Community Manager 

Android & WoA working groups

[Bertrand Marquis]

Hi Nathalie,

I was on leave and back today as I informed Julianus and others.
So i will be available on the 7/20 (today) and on the 7/27 as well.

Cheers
Bertrand

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