Commit 01b71917 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by Gleb Natapov

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
......@@ -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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