[PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock

["Michael S. Tsirkin" <mst@redhat.com>]

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().

Unsurprisingly, the gain is small but measureable using the unit test
microbenchmark:
before
	vmcall 1407
after
	vmcall 1357

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

--

I didn't stress test this yet, sending out for early review/flames.

Paul, could you review this patch please?
Documentation/memory-barriers.txt says that unlock has a weaker
uni-directional barrier, but in practice srcu_read_unlock calls
smp_mb().

Is it OK to rely on this? If not, can I add
smp_mb__after_srcu_read_unlock (making it an empty macro for now)
so we can avoid an actual extra smp_mb()?

Thanks.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8617c9d..a48fb36 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5949,8 +5949,10 @@ restore:
 
 	/* We should set ->mode before check ->requests,
 	 * see the comment in make_all_cpus_request.
+	 *
+	 * srcu_read_unlock below acts as a memory barrier.
 	 */
-	smp_mb();
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 
 	local_irq_disable();
 
@@ -5960,12 +5962,11 @@ restore:
 		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);
 

-- 
MST