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

Message ID 20240522001817.619072-18-dwmw2@infradead.org (mailing list archive)
State New, archived
Headers show
Series Cleaning up the KVM clock mess | expand

Commit Message

David Woodhouse May 22, 2024, 12:17 a.m. UTC
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.

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(-)

Comments

Paul Durrant May 24, 2024, 2:14 p.m. UTC | #1
On 22/05/2024 01:17, 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.
> 
> 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(-)
> 

Reviewed-by: Paul Durrant <paul@xen.org>
Sean Christopherson Aug. 16, 2024, 4:28 a.m. UTC | #2
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
>
diff mbox series

Patch

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