Re: [RFC PATCH 0/4] vmclock: add support for VM generation counter and notifications

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

On Mon, Dec 01, 2025 at 12:50:24PM +0000, Chalios, Babis wrote:
> Latest specification of VMClock[1] adds support for VM generation counter
> and notifications. VM generation counter is similar to disruption_marker
> but it only changes when the guest has been loaded from a snapshot, not
> on live migration. Its purpose is to notify the guest about snapshot
> events and let it perform actions such as recreating UUIDs, resetting
> network connections, reseeding entropy, etc.
> 
> Moreover, the spec now describes a notification that the device can send
> after updating the seq counter to a new even number.
> 
> I have already sent the Linux changes to the mailing list here:
> https://lore.kernel.org/lkml/20251127103159.19816-1-bchalios@amazon.es/T/#u
> 
> [1] https://david.woodhou.se/VMClock.pdf

Should that spec document the expected behaviour of guests when a hypervisor
advertizes both vmclock and vmgenid devices ?

QEMU supports both, and to avoid assumptions about whether a guest supports
the newer vmclock, I could expect mgmt apps to expose both these QEMU
devices.

IIUC, your intent is that 'vmclock' obsoletes the need for 'vmgenid', so
should the spec say that explicitly, and suggest that guest kernels ignore
the vmgenid if both  are present, to avoid the same kind of actions being
triggered twice ?


With regards,
Daniel
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