Re: [RFC PATCH v3 18/21] KVM: x86: Avoid gratuitous global clock reload in kvm_arch_vcpu_load()

[Paul Durrant]

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> Commit d98d07ca7e034 ("KVM: x86: update pvclock area conditionally, on
> cpu migration") turned an unconditional KVM_REQ_CLOCK_UPDATE into a
> conditional one, if either the master clock isn't enabled *or* the vCPU
> was not previously scheduled (vcpu->cpu == -1). The commit message doesn't
> explain the latter condition, which is specifically for the master clock
> case.
>
> Commit 0061d53daf26f ("KVM: x86: limit difference between kvmclock
> updates") later turned that into a KVM_REQ_GLOBAL_CLOCK_UPDATE to avoid
> skew between vCPUs.
>
> In master clock mode there is no need for any of that, regardless of
> whether/where this vCPU was previously scheduled.
>
> Do it only if (!kvm->arch.use_master_clock).
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>   arch/x86/kvm/x86.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>