Re: [RFC PATCH v3 17/21] KVM: x86: Avoid global clock update on setting KVM clock MSR

[Sean Christopherson <seanjc@google.com>]

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Commit 0061d53daf26 ("KVM: x86: limit difference between kvmclock updates")
> introduced a KVM_REQ_GLOBAL_CLOCK_UPDATE when one vCPU set up its clock.
> 
> This was a workaround for the ever-drifting clocks which were based on the
> host's CLOCK_MONOTONIC and thus subject to NTP skew. On booting or resuming
> a guest, it just leads to running kvm_guest_time_update() twice for each
> vCPU for now good reason.

s/now/no

> Just use KVM_REQ_CLOCK_UPDATE on the vCPU itself, and only in the case
> where the KVM clock is being set up, not turned off.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  arch/x86/kvm/x86.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 437412b36cae..32a873d5ed00 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2361,13 +2361,13 @@ static void kvm_write_system_time(struct kvm_vcpu *vcpu, gpa_t system_time,
>  	}
>  
>  	vcpu->arch.time = system_time;
> -	kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
>  
>  	/* we verify if the enable bit is set... */
> -	if (system_time & 1)
> +	if (system_time & 1) {
>  		kvm_gpc_activate(&vcpu->arch.pv_time, system_time & ~1ULL,
>  				 sizeof(struct pvclock_vcpu_time_info));
> -	else
> +		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +	} else

Needs curly braces now that the if-statement has 'em.

>  		kvm_gpc_deactivate(&vcpu->arch.pv_time);
>  
>  	return;
> -- 
> 2.44.0
>