2024-05-22 01:27:18

by David Woodhouse

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

From: David Woodhouse <[email protected]>

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 <[email protected]>
---
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
kvm_gpc_deactivate(&vcpu->arch.pv_time);

return;
--
2.44.0

2024-05-24 14:15:04

by Paul Durrant

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

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <[email protected]>
>
> 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 <[email protected]>
> ---
> arch/x86/kvm/x86.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>

Reviewed-by: Paul Durrant <[email protected]>