Re: [PATCH kvmtool v2 4/6] arm64: Add counter offset control

[Marc Zyngier <maz@kernel.org>]

On Fri, 25 Jul 2025 15:40:58 +0100,
Andre Przywara <andre.przywara@arm.com> wrote:
> 
> From: Marc Zyngier <maz@kernel.org>
> 
> KVM now allows to offset the arch timer counter values reported to a
> guest, via a per-VM ioctl. This is conceptually similar to the effects
> of CNTVOFF_EL2, but applies to both the emulated physical and virtual
> counter, and also to all VCPUs.

Not really. Architecturally, there is only a single, global counter,
and multiple *views* of that counter.

What this option does is to offset the global counter. Then, the
various per-CPU offsets are applied on the views that can be affected
by them. This is cumulative. So for example, CNTVCT_EL0 is affected by
both the global offset and CNTVOFF_EL2 when read from EL0, EL1, and
EL2 when E2H==0. Behind the scenes, this is of course implemented with
CNTVOFF_EL2 (when it works), but that's not really relevant here.

I'd suggest something along the lines of:

"KVM allows the offsetting of the global counter in order to help with
 migration of a VM. This offset applies cumulatively with the offsets
 provided by the architecture.

 Although kvmtool doesn't provide a way to migrate a VM, controlling
 this offset is useful to test the timer subsystem."

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.