Re: [PATCH 08/16] KVM: arm64: timers: Allow userspace to set the counter offsets

[Marc Zyngier <maz@kernel.org>]

On Wed, 08 Mar 2023 07:53:46 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> > I certainly agree that (2a) is highly desirable to get existing VMMs to
> > 'do the right thing' for free. Playing devil's advocate, would this not
> > also break the tracing example you've given of correlating timestamps
> > between the host and guest? I wouldn't expect a userspace + VM tracing
> > contraption to live migrate but restoring from a snapshot seems
> > plausible.
> 
> The problem I'm alluding to here is that the VMM will save/restore
> the physical counter value and cause KVM to offset the physical counter.
> Live migration is a pretty obvious example, but resuming from a snapshot
> after resetting a system be similarly affected.

My take on this is that if you have produced the snapshot on a
pre-CNTPCT host, there will be no change in behaviour. If you've
produced the snapshot on a new host, you get the new behaviour.

I am willing to be accommodating to the use case, but only to a
certain extent! ;-)

	M.

-- 
Without deviation from the norm, progress is not possible.