Re: [PATCH v2 05/11] KVM: x86/xen: Use guest's copy of pvclock when starting timer

[Paul Durrant]

On 01/02/2025 01:38, Sean Christopherson wrote:
> Use the guest's copy of its pvclock when starting a Xen timer, as KVM's
> reference copy may not be up-to-date, i.e. may yield a false positive of
> sorts.  In the unlikely scenario that the guest is starting a Xen timer
> and has used a Xen pvclock in the past, but has since but turned it "off",
> then vcpu->arch.hv_clock may be stale, as KVM's reference copy is updated
> if and only if at least one pvclock is enabled.
>
> Furthermore, vcpu->arch.hv_clock is currently used by three different
> pvclocks: kvmclock, Xen, and Xen compat.  While it's extremely unlikely a
> guest would ever enable multiple pvclocks, effectively sharing KVM's
> reference clock could yield very weird behavior.  Using the guest's active
> Xen pvclock instead of KVM's reference will allow dropping KVM's
> reference copy.
>
> Fixes: 451a707813ae ("KVM: x86/xen: improve accuracy of Xen timers")
> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/xen.c | 65 ++++++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 60 insertions(+), 5 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>