linux-kernel - [RFC PATCH v3 19/21] KVM: x86: Avoid periodic KVM clock updates in master clock mode

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Date: Wed, 22 May 2024 01:17:14 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Paul Durrant <paul@....org>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	jalliste@...zon.co.uk,
	sveith@...zon.de,
	zide.chen@...el.com,
	Dongli Zhang <dongli.zhang@...cle.com>,
	Chenyi Qiang <chenyi.qiang@...el.com>
Subject: [RFC PATCH v3 19/21] KVM: x86: Avoid periodic KVM clock updates in master clock mode

From: David Woodhouse <dwmw@...zon.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@...zon.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