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From: David Woodhouse <dwmw2@infradead.org>
To: kvm@vger.kernel.org
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Subject: [RFC PATCH v3 18/21] KVM: x86: Avoid gratuitous global clock reload in kvm_arch_vcpu_load()
Date: Wed, 22 May 2024 01:17:13 +0100
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From: David Woodhouse <dwmw@amazon.co.uk>

Commit d98d07ca7e034 ("KVM: x86: update pvclock area conditionally, on
cpu migration") turned an unconditional KVM_REQ_CLOCK_UPDATE into a
conditional one, if either the master clock isn't enabled *or* the vCPU
was not previously scheduled (vcpu->cpu == -1). The commit message doesn't
explain the latter condition, which is specifically for the master clock
case.

Commit 0061d53daf26f ("KVM: x86: limit difference between kvmclock
updates") later turned that into a KVM_REQ_GLOBAL_CLOCK_UPDATE to avoid
skew between vCPUs.

In master clock mode there is no need for any of that, regardless of
whether/where this vCPU was previously scheduled.

Do it only if (!kvm->arch.use_master_clock).

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 32a873d5ed00..dd53860ca284 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5161,7 +5161,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * On a host with synchronized TSC, there is no need to update
 		 * kvmclock on vcpu->cpu migration
 		 */
-		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
+		if (!vcpu->kvm->arch.use_master_clock)
 			kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 		if (vcpu->cpu != cpu)
 			kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
-- 
2.44.0