[PATCH] KVM: x86: optimize delivery of TSC deadline timer interrupt

[Paolo Bonzini]

The newly-added tracepoint shows the following results on
the tscdeadline_latency test:

        qemu-kvm-8387  [002]  6425.558974: kvm_vcpu_wakeup:      poll time 10407 ns
        qemu-kvm-8387  [002]  6425.558984: kvm_vcpu_wakeup:      poll time 0 ns
        qemu-kvm-8387  [002]  6425.561242: kvm_vcpu_wakeup:      poll time 10477 ns
        qemu-kvm-8387  [002]  6425.561251: kvm_vcpu_wakeup:      poll time 0 ns

and so on.  This is because we need to go through kvm_vcpu_block again
after the timer IRQ is injected.  Avoid it by polling once before
entering kvm_vcpu_block.

On my machine (Xeon E5 Sandy Bridge) this removes about 500 cycles (7%)
from the latency of the TSC deadline timer.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0b8dd13676ef..1e766033ebff 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6389,11 +6389,15 @@ static inline int __vcpu_run(struct kvm *kvm, struct kvm_vcpu *vcpu)
 	    !vcpu->arch.apf.halted)
 		return vcpu_enter_guest(vcpu);
 
-	srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
-	kvm_vcpu_block(vcpu);
-	vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
-	if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
-		return 1;
+	if (kvm_arch_vcpu_runnable(vcpu))
+		clear_bit(KVM_REQ_UNHALT, &vcpu->requests);
+	else {
+		srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
+		kvm_vcpu_block(vcpu);
+		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
+		if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
+			return 1;
+	}
 
 	kvm_apic_accept_events(vcpu);
 	switch(vcpu->arch.mp_state) {
-- 
1.8.3.1

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