Re: [PATCH 0/2] vmgenid: add generation counter

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Aug 03, 2022 at 03:41:45PM +0200, bchalios@amazon.es wrote:
> From: Babis Chalios <bchalios@amazon.es>
> 
> VM generation ID exposes a GUID inside the VM which changes every time a
> VM restore is happening. Typically, this GUID is used by the guest
> kernel to re-seed its internal PRNG. As a result, this value cannot be
> exposed in guest user-space as a notification mechanism for VM restore
> events.
> 
> This patch set extends vmgenid to introduce a 32 bits generation counter
> whose purpose is to be used as a VM restore notification mechanism for
> the guest user-space.
> 
> It is true that such a counter could be implemented entirely by the
> guest kernel, but this would rely on the vmgenid ACPI notification to
> trigger the counter update, which is inherently racy. Exposing this
> through the monitor allows the updated value to be in-place before
> resuming the vcpus, so interested user-space code can (atomically)
> observe the update without relying on the ACPI notification.

The VM generation ID feature in QEMU is implementing a spec defined
by Microsoft. It is implemented in HyperV, VMWare, QEMU and possibly
more. This series is proposing a QEMU specific variant, which means
Linux running on all these other hypervisor platforms won't benefit
from the change. If the counter were provided entirely in the guest
kernel, then it works across all hypervisors.

It feels like the kernel ought to provide an implementation itself
as a starting point, with this QEMU change merely being an optional
enhancement to close the race window.

Ideally there would be someone at Microsoft we could connect with to
propose they include this feature in a VM Gen ID spec update, but I
don't personally know who to contact about that kind of thing. A
spec update would increase chances that this change gets provieded
across all hypervisors.

With regards,
Daniel
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