Sandbox::https://github.com/cloud-hypervisor/cloud-hypervisor/pull/8907

saravan2 · 2026-09-22T08:08:10Z

Adds a default VM Generation ID device. The guest kernel requires CONFIG_VMGENID to get notified and reseed the random number generator.
The 128 bits value lives in a page the device owns, mapped into the guest at an address the platform MMIO allocator assigns. The location is recorded in the device tree as an MMIO address range, so it survives a snapshot and a restore sequence can access it.
The spec permits device MMIO, but the Linux vmgenid driver implementation reads it with a plain memcpy, which becomes a load pair on arm64. arm64 does not offer KVM syndrome for load pairs so it wont be able to emulate it. For this reason the page is backed by a KVM memslot.
Live migration generates a new VM generation id, because the destination completes the migration through the restore path.
The change notification uses the existing GED interrupt.
Tests :
- unit test, test_vmgenid_regenerate, covers value regeneration.
- integration test, test_snapshot_restore_vmgenid, snapshots and restores a VM and asserts the GED interrupt fired and the guest reseeded, on both arches with arm64 using acpi=on.

rbradford

I think we can make it unconditional - if the guest is enlightened to use it it will use it.

rbradford

The spec says it can be device MMIO as well as RAM. Could we do that instead?

saravan2 · 2026-09-22T17:05:22Z

I think we can make it unconditional - if the guest is enlightened to use it it will use it.

Yay. I made it optional to get through upstream review.

rbradford · 2026-09-22T17:20:06Z

I think we can make it unconditional - if the guest is enlightened to use it it will use it.

Yay. I made it optional to get through upstream review.

Adding an option is a sure fire way to get me to give a "Reqeust changes" review :-)

rbradford · 2026-09-23T08:00:56Z

The spec says it can be device MMIO as well as RAM. Could we do that instead?

Maybe just piggy back onto the existing ACPI MMIO device rather than needing to add a new one?

phip1611 · 2026-09-25T06:11:43Z

The spec says it can be device MMIO as well as RAM. Could we do that instead?

Maybe just piggy back onto the existing ACPI MMIO device rather than needing to add a new one?

You mean DeviceManager which implements BusDevice? I wanted to split DeviceManager into its true core and AcpiPciHotplugController already to keep the code maintainable and prevent a possible race condition (#8028). So please don't introduce another functionality into this gigantic type. I am advocating for a dedicated device, using the MMIO path as Rob pointed out.

rbradford · 2026-09-25T09:23:09Z

The spec says it can be device MMIO as well as RAM. Could we do that instead?

Maybe just piggy back onto the existing ACPI MMIO device rather than needing to add a new one?

You mean DeviceManager which implements BusDevice? I wanted to split DeviceManager into its true core and AcpiPciHotplugController already to keep the code maintainable and prevent a possible race condition (#8028). So please don't introduce another functionality into this gigantic type. I am advocating for a dedicated device, using the MMIO path as Rob pointed out.

I'd forgotten that ACPI shutdown and hotplug were separate MMIO devices now. Indeed it doesn't make sense to lump into either of those. But BusDevice implementations are cheap and easy so we can have a new one.

saravan2 · 2026-09-25T09:34:23Z

I think we can make it unconditional - if the guest is enlightened to use it it will use it.

Yay. I made it optional to get through upstream review.

Adding an option is a sure fire way to get me to give a "Reqeust changes" review :-)

The device is unconditional now, and --platform no longer has a vmgenid key.

saravan2 · 2026-09-25T09:37:06Z

The spec says it can be device MMIO as well as RAM. Could we do that instead?

Maybe just piggy back onto the existing ACPI MMIO device rather than needing to add a new one?

You mean DeviceManager which implements BusDevice? I wanted to split DeviceManager into its true core and AcpiPciHotplugController already to keep the code maintainable and prevent a possible race condition (#8028). So please don't introduce another functionality into this gigantic type. I am advocating for a dedicated device, using the MMIO path as Rob pointed out.

VmGenIdDevice is its own type with its own page, and nothing was added to DeviceManager's BusDevice implementation.

saravan2 · 2026-09-25T09:41:47Z

The spec says it can be device MMIO as well as RAM. Could we do that instead?

Maybe just piggy back onto the existing ACPI MMIO device rather than needing to add a new one?

The value now lives in a page the device owns, mapped into the guest at an address from the platform MMIO allocator. So there is no guest RAM carve out and no layout constant.

yamahata · 2026-09-25T23:55:13Z

Now it became much better.
It's good to carve out the region from MMIO region and transfer the address on migration.
Lastly could you please include a pointer to the spec?

rbradford

Unfortunately I think you missed the point of putting on the MMIO bus.

rbradford

Add an integration test? The unit tests have too many excessive helpers.

rbradford · 2026-09-27T10:34:27Z

I also strongly suspect you used Claude to write the PR ("Where the value lives", "land", "draws", "closes the gap", etc). Please don't do that it's disrespectful to the human reviewers.

saravan2 · 2026-09-27T18:05:15Z

Add an integration test? The unit tests have too many excessive helpers.

Our CI kernel (ch-release-v6.16.9-20260508) does not have CONFIG_VMGENID turned on :
https://github.com/cloud-hypervisor/linux
To create this integration test, we would require a new kernel release and an update to scripts/test_assets.yaml

rbradford

Thank you - looking good! I think the page size is a blocker and I would love it if we could push some complexity out from device manager into the device.

rbradford

Comments in the integration tests are a bit wordy. Otherwise lgtm! Thanks for the patience. This will be a nice improvement.

likebreath

@saravan2 Can you address the last comment above and rebase? It would be good to have it for v54.0.

saravan2 · 2026-10-09T01:06:20Z

Addressed final review suggestions. Thanks a ton.

likebreath · 2026-10-09T06:21:51Z

@rbradford @rhakobyan The SEV-SNP job has been failing twice in a roll from the MQ. Can you please take a look? Thank you!

rhakobyan · 2026-10-09T09:32:49Z

@rbradford @rhakobyan The SEV-SNP job has been failing twice in a roll from the MQ. Can you please take a look? Thank you!

@likebreath ch-linux kernel is missing this commit which is needed for VMGENID to work under SEV-SNP. I manually backported it onto the SEV-SNP worker and the tests now pass. But we probably want to backport it to the upstream ch-linux as well.

rbradford · 2026-10-09T10:31:20Z

@rbradford @rhakobyan The SEV-SNP job has been failing twice in a roll from the MQ. Can you please take a look? Thank you!

@likebreath ch-linux kernel is missing this commit which is needed for VMGENID to work under SEV-SNP. I manually backported it onto the SEV-SNP worker and the tests now pass. But we probably want to backport it to the upstream ch-linux as well.

I'm on it!

saravan2 · 2026-10-09T22:55:47Z

@rbradford, @likebreath, @phip1611

test_snapshot_restore_vmgenid passes on integration-arm64

        PASS [  45.738s] (13/19) cloud-hypervisor::integration ivshmem::test_snapshot_restore_vmgenid

Once the underlying kernel used in integration-sev-snp is patched, this PR should clear the merge queue. I hope this happens before v54 release.

tutkodev

Tested-by: Martin Tutko martin@tutko.dev

Tested 446a2db against its merge base 125bd3d on x86_64 KVM. Guest: Linux 7.2.9 with CONFIG_VMGENID=y, direct kernel boot, 2 vCPU, 2 GiB. One snapshot per VMM, taken at guest uptime 40 s. In each guest the test checks three things: VMGENCTR under /sys/bus/acpi/devices, the GED interrupt count in /proc/interrupts, and "crng reseeded due to virtual machine fork" in dmesg.

Case	`446a2db`	`125bd3d`
Cold boot	device present, GED 0, no reseed	no device
Restore, `memory_restore_mode=copy`, 10 clones	10/10: GED 1, reseeded	0/10
Restore, `memory_restore_mode=copyonwrite`, 10 clones	10/10: GED 1, reseeded	0/10
Pause/resume of a restored VM	no new GED, no new reseed	no device
Snapshot of a restored VM, then restore	GED 2, reseeded twice	no device
Live migration of a restored VM (same host, unix socket)	destination: GED 2, reseeded twice	no device

The last three rows are not covered by test_snapshot_restore_vmgenid.

The same four commits, applied on main 3e32289 together with #9035 and an unrelated two-line local change, have also run as the VMM for up to 200 concurrent guests whose kernels lack the vmgenid driver. No regressions were observed.

rbradford · 2026-10-10T07:17:03Z

@rbradford, @likebreath, @phip1611

test_snapshot_restore_vmgenid passes on integration-arm64
        PASS [  45.738s] (13/19) cloud-hypervisor::integration ivshmem::test_snapshot_restore_vmgenid
Once the underlying kernel used in integration-sev-snp is patched, this PR should clear the merge queue. I hope this happens before v54 release.

If you rebase you should be able to get the kernel update.

The vmgenid driver binds to an ACPI device carrying a 128 bit identifier and reseeds the kernel random number generator whenever that identifier changes. Add the device model and the AML the driver expects, which is a hardware ID it matches on and an ADDR package holding the low and high halves of its address. The device writes the value into a region it owns rather than into guest RAM. Assisted-by: Claude [Claude Code] Signed-off-by: Saravanan D <saravanand@crusoe.ai>

Every VM restored from a snapshot starts with the guest random number generator state that snapshot captured, so two clones of one template produce the same stream from /dev/urandom until something reseeds them. Give the device a page of its own, mapped into the guest at an address from the platform allocator, and write a fresh value into it on every restore. One snapshot can be restored any number of times and the VMM cannot tell a clone from an ordinary resume, so every restore is treated as a fork. The change notification uses the existing GED interrupt. The VM Generation ID specification permits the value in device MMIO, but the Linux driver does not read it that way. It uses a plain memcpy through a memremap mapping rather than the MMIO accessors, which on arm64 compiles to a load pair. The arm64 fault for a load pair reports no instruction syndrome, so KVM cannot emulate the access and a trapping region would fail there. Backing the page with a memory slot keeps the read a normal memory access on amd64 and arm64 architectures. The address is recorded in the device tree, so a restored VM reuses it rather than allocating again. An incoming live migration completes through the same restore path, so it writes a new value as well. Assisted-by: Claude [Claude Code] Signed-off-by: Saravanan D <saravanand@crusoe.ai>

Describe the device, the guest CONFIG_VMGENID requirement, how a direct boot reaches ACPI on x86 and aarch64, and the reseed on restore and on live migration. The reseed reaches the kernel generator only, so a program that seeded a generator of its own is left untouched. Assisted-by: Claude [Claude Code] Signed-off-by: Saravanan D <saravanand@crusoe.ai>

Boot a VM, snapshot it, restore it, and confirm the restore signalled the generation ID change. The restored guest must show the GED interrupt fired and its kernel RNG reseeded through the vmgenid driver. The test runs on x86 and aarch64. On aarch64 the guest reaches ACPI on a direct boot through the EFI stub tables and prefers it over the device tree with acpi=on. Assisted-by: Claude [Claude Code] Signed-off-by: Saravanan D <saravanand@crusoe.ai>

saravan2 · 2026-10-10T20:03:00Z

@rbradford Rebased.

rbradford · 2026-10-11T10:34:07Z

@saravan2 We dropped the Assisted-by requirement.

rbradford

Did you ask the LLM to add the trait or did it do that on its own accord to the prompt of needing to free the memory? It's not pattern we have elsewhere in the project for the single user like this.

rbradford · 2026-10-11T10:42:42Z

+}
+
+#[cfg(not(target_arch = "riscv64"))]
+impl devices::VmGenIdOps for VmGenIdHandler {


I don't like the addition of this trait for just one use (and it doesn't match anything else we do in the codebase). You could directly code the region setup into add_vmgenid_device() which is already busy with the MMIO allocation and the release into DeviceManager::drop().

A good abstraction, that i'm not proposing you add to this PR. Would be the idea of an OwnedMemoryRegion that would handle its own release.

saravan2 self-assigned this Sep 22, 2026

saravan2 marked this pull request as ready for review September 22, 2026 08:18

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eliranw mentioned this pull request Sep 22, 2026

ateom-microvm: reseed the guest CRNG on restore agent-substrate/substrate#1524

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build: Release v54.0 #9010

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