From: David Woodhouse <dwmw@amazon.co.uk>

When the KVM clock is in master clock mode, updating the KVM clock is
pointless. Let the periodic work 'expire', and start it running again
from kvm_end_pvclock_update() if the master clock mode is ever turned
off again.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/x86.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dd53860ca284..10b82f1b110d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -158,6 +158,8 @@ module_param(min_timer_period_us, uint, 0644);
 static bool __read_mostly kvmclock_periodic_sync = true;
 module_param(kvmclock_periodic_sync, bool, 0444);
 
+#define KVMCLOCK_SYNC_PERIOD (300 * HZ)
+
 /* tsc tolerance in parts per million - default to 1/2 of the NTP threshold */
 static u32 __read_mostly tsc_tolerance_ppm = 250;
 module_param(tsc_tolerance_ppm, uint, 0644);
@@ -3187,6 +3189,10 @@ static void kvm_end_pvclock_update(struct kvm *kvm)
 	kvm_for_each_vcpu(i, vcpu, kvm)
 		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 
+	if (kvmclock_periodic_sync && !kvm->arch.use_master_clock)
+		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
+				      KVMCLOCK_SYNC_PERIOD);
+
 	/* guest entries allowed */
 	kvm_for_each_vcpu(i, vcpu, kvm)
 		kvm_clear_request(KVM_REQ_MCLOCK_INPROGRESS, vcpu);
@@ -3555,8 +3561,6 @@ static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
 					KVMCLOCK_UPDATE_DELAY);
 }
 
-#define KVMCLOCK_SYNC_PERIOD (300 * HZ)
-
 static void kvmclock_sync_fn(struct work_struct *work)
 {
 	struct delayed_work *dwork = to_delayed_work(work);
@@ -3564,6 +3568,9 @@ static void kvmclock_sync_fn(struct work_struct *work)
 					   kvmclock_sync_work);
 	struct kvm *kvm = container_of(ka, struct kvm, arch);
 
+	if (!kvm->arch.use_master_clock)
+		return;
+
 	schedule_delayed_work(&kvm->arch.kvmclock_update_work, 0);
 	schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
 					KVMCLOCK_SYNC_PERIOD);
@@ -12551,7 +12558,8 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 
 	mutex_unlock(&vcpu->mutex);
 
-	if (kvmclock_periodic_sync && vcpu->vcpu_idx == 0)
+	if (kvmclock_periodic_sync && !kvm->arch.use_master_clock &&
+	    vcpu->vcpu_idx == 0)
 		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
 						KVMCLOCK_SYNC_PERIOD);
 }
-- 
2.44.0

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> When the KVM clock is in master clock mode, updating the KVM clock is
> pointless.

Please explain why it's pointless.  I almost feel bad asking you to elaborate
since you've written wonderful changelogs for so many other patches, but far too
many of the (historical) kvmclock commits assume the reader is some omniscient
being, and it's definitely contributed to things getting into such a mess state.

> Let the periodic work 'expire', and start it running again
> from kvm_end_pvclock_update() if the master clock mode is ever turned
> off again.

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> When the KVM clock is in master clock mode, updating the KVM clock is
> pointless. Let the periodic work 'expire', and start it running again
> from kvm_end_pvclock_update() if the master clock mode is ever turned
> off again.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   arch/x86/kvm/x86.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
> 

Reviewed-by: Paul Durrant <paul@xen.org>