kvm: optimize out smp_mb after srcu_read_unlock (01b71917) · Commits · 戴 / test

I noticed that srcu_read_lock/unlock both have a memory barrier, so just by moving srcu_read_unlock earlier we can get rid of one call to smp_mb() using smp_mb__after_srcu_read_unlock instead. Unsurprisingly, the gain is small but measureable using the unit test microbenchmark: before vmcall in the ballpark of 1410 cycles after vmcall in the ballpark of 1360 cycles Signed-off-by:

Michael S. Tsirkin <mst@redhat.com> Signed-off-by:

Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/x86.c

+4 −3

Original line number	Diff line number	Diff line
		@@ -5966,10 +5966,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)

		vcpu->mode = IN_GUEST_MODE;

		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);

		/* We should set ->mode before check ->requests,
		* see the comment in make_all_cpus_request.
		*/
		smp_mb();
		smp_mb__after_srcu_read_unlock();

		local_irq_disable();

		@@ -5979,12 +5981,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
		smp_wmb();
		local_irq_enable();
		preempt_enable();
		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
		r = 1;
		goto cancel_injection;
		}

		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);

		if (req_immediate_exit)
		smp_send_reschedule(vcpu->cpu);

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