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kvm: optimize out smp_mb after srcu_read_unlock

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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>
This commit is contained in:
Michael S. Tsirkin 2013-11-04 22:36:25 +02:00 committed by Gleb Natapov
parent ce332f662d
commit 01b71917b5
1 changed files with 4 additions and 3 deletions
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7
arch/x86/kvm/x86.c
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@ -5966,10 +5966,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
vcpu->mode = IN_GUEST_MODE; vcpu->mode = IN_GUEST_MODE;
srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
/* We should set ->mode before check ->requests, /* We should set ->mode before check ->requests,
* see the comment in make_all_cpus_request. * see the comment in make_all_cpus_request.
*/ */
smp_mb(); smp_mb__after_srcu_read_unlock();
local_irq_disable(); local_irq_disable();
@ -5979,12 +5981,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
smp_wmb(); smp_wmb();
local_irq_enable(); local_irq_enable();
preempt_enable(); preempt_enable();
vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
r = 1; r = 1;
goto cancel_injection; goto cancel_injection;
} }
srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
if (req_immediate_exit) if (req_immediate_exit)
smp_send_reschedule(vcpu->cpu); smp_send_reschedule(vcpu->cpu);