Re: [PATCH v2] hw/virtio: reset virtio balloon stats on machine reset

[David Hildenbrand <david@redhat.com>]

On 04.02.25 10:42, Daniel P. Berrangé wrote:
> When a machine is first booted, all virtio balloon stats are initialized
> to their default value -1 (18446744073709551615 when represented as
> unsigned).
> 
> They remain that way while the firmware is loading, and early phase of
> guest OS boot, until the virtio-balloon driver is activated. Thereafter
> the reported stats reflect the guest OS activity.
> 
> When a machine reset is performed, however, the virtio-balloon stats are
> left unchanged by QEMU, despite the guest OS no longer updating them,
> nor indeed even still existing.
> 
> IOW, the mgmt app keeps getting stale stats until the guest OS starts
> once more and loads the virtio-balloon driver (if ever). At that point
> the app will see a discontinuity in the reported values as they sudden
> jump from the stale value to the new value. This jump is indigituishable
> from a valid data update.
> 
> While there is an "last-updated" field to report on the freshness of
> the stats, that does not unambiguously tell the mgmt app whether the
> stats are still conceptually relevant to the current running workload.
> 
> It is more conceptually useful to reset the stats to their default
> values on machine reset, given that the previous guest workload the
> stats reflect no longer exists. The mgmt app can now clearly identify
> that there are is no stats information available from the current
> executing workload.
> 
> The 'last-updated' time is also reset back to 0.
> 
> IOW, on every machine reset, the virtio stats are in the same clean
> state they were when the macine first powered on.
> 
> A functional test is added to validate this behaviour with a real
> world guest OS.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb