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Commit 01b71917 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by Gleb Natapov
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kvm: optimize out smp_mb after srcu_read_unlock 



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: default avatarMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: default avatarGleb Natapov <gleb@redhat.com>
parent ce332f66

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);