On Wed, May 22, 2024, 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.
It handles the first load of the vCPU, which is distinctly different from CPU
migration.
> 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(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 32a873d5ed00..dd53860ca284 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5161,7 +5161,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> * On a host with synchronized TSC, there is no need to update
> * kvmclock on vcpu->cpu migration
> */
> - if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
> + if (!vcpu->kvm->arch.use_master_clock)
> kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
I believe this needs to be:
if (!vcpu->kvm->arch.use_master_clock)
kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
else if (vcpu->cpu == -1)
kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
or alternatively do KVM_REQ_CLOCK_UPDATE during vCPU creation (hotplug?).
> if (vcpu->cpu != cpu)
> kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
> --
> 2.44.0
>
