Re: [PATCH v3 0/4] ptp: vmclock: Add VM generation counter and ACPI notification

[Paolo Abeni]

On 12/3/25 1:35 PM, Chalios, Babis wrote:
> Similarly to live migration, starting a VM from some serialized state
> (aka snapshot) is an event which calls for adjusting guest clocks, hence
> a hypervisor should increase the disruption_marker before resuming the
> VM vCPUs, letting the guest know.
> 
> However, loading a snapshot, is slightly different than live migration,
> especially since we can start multiple VMs from the same serialized
> state. Apart from adjusting clocks, the guest needs to take additional
> action during such events, e.g. recreate UUIDs, reset network
> adapters/connections, reseed entropy pools, etc. These actions are not
> necessary during live migration. This calls for a differentiation
> between the two triggering events.
> 
> We differentiate between the two events via an extra field in the
> vmclock_abi, called vm_generation_counter. Whereas hypervisors should
> increase the disruption marker in both cases, they should only increase
> vm_generation_counter when a snapshot is loaded in a VM (not during live
> migration).
> 
> Additionally, we attach an ACPI notification to VMClock. Implementing
> the notification is optional for the device. VMClock device will declare
> that it implements the notification by setting
> VMCLOCK_FLAG_NOTIFICATION_PRESENT bit in vmclock_abi flags. Hypervisors
> that implement the notification must send an ACPI notification every
> time seq_count changes to an even number. The driver will propagate
> these notifications to userspace via the poll() interface.

Linux tagged 6.18 final, so net-next is closed for new code submissions
per the announcement at
https://lore.kernel.org/20251130174502.3908e3ee@xxxxxxxxxx

/P