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

[Sean Christopherson]

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
>