KVM: Avoid atomic operations when kicking the running vCPU

If we do have the vcpu mutex, as is the case if kvm_running_vcpu is set
to the target vcpu of the kick, changes to vcpu->mode do not need atomic
operations; cmpxchg is only needed _outside_ the mutex to ensure that
the IN_GUEST_MODE->EXITING_GUEST_MODE change does not race with the vcpu
thread going OUTSIDE_GUEST_MODE.

Use this to optimize the case of a vCPU sending an interrupt to itself.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f93b601..e9990c4 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3396,6 +3396,19 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
        if (kvm_vcpu_wake_up(vcpu))
                return;
 
+       me = get_cpu();
+       /*
+        * The only state change done outside the vcpu mutex is IN_GUEST_MODE
+        * to EXITING_GUEST_MODE.  Therefore the moderately expensive "should
+        * kick" check does not need atomic operations if kvm_vcpu_kick is used
+        * within the vCPU thread itself.
+        */
+       if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
+               if (vcpu->mode == IN_GUEST_MODE)
+                       WRITE_ONCE(vcpu->mode, EXITING_GUEST_MODE);
+               goto out;
+       }
+
        /*
         * Note, the vCPU could get migrated to a different pCPU at any point
         * after kvm_arch_vcpu_should_kick(), which could result in sending an
@@ -3403,12 +3416,12 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
         * IPI is to force the vCPU to leave IN_GUEST_MODE, and migrating the
         * vCPU also requires it to leave IN_GUEST_MODE.
         */
-       me = get_cpu();
        if (kvm_arch_vcpu_should_kick(vcpu)) {
                cpu = READ_ONCE(vcpu->cpu);
                if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
                        smp_send_reschedule(cpu);
        }
+out:
        put_cpu();
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_kick);