KVM: Get rid of KVM_REQ_KICK

KVM_REQ_KICK poisons vcpu->requests by having a bit set during normal
operation.  This causes the fast path check for a clear vcpu->requests
to fail all the time, triggering tons of atomic operations.

Fix by replacing KVM_REQ_KICK with a vcpu->guest_mode atomic.

Signed-off-by: Avi Kivity <avi@redhat.com>

2 files changed, 11 insertions, 7 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 63c87adcec48..fc5611b4007f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4604,13 +4604,15 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
         if (vcpu->fpu_active)
                 kvm_load_guest_fpu(vcpu);
 
-        local_irq_disable();
+        atomic_set(&vcpu->guest_mode, 1);
+        smp_wmb();
 
-        clear_bit(KVM_REQ_KICK, &vcpu->requests);
-        smp_mb__after_clear_bit();
+        local_irq_disable();
 
-        if (vcpu->requests || need_resched() || signal_pending(current)) {
-                set_bit(KVM_REQ_KICK, &vcpu->requests);
+        if (!atomic_read(&vcpu->guest_mode) || vcpu->requests
+            || need_resched() || signal_pending(current)) {
+                atomic_set(&vcpu->guest_mode, 0);
+                smp_wmb();
                 local_irq_enable();
                 preempt_enable();
                 r = 1;
@@ -4655,7 +4657,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
         if (hw_breakpoint_active())
                 hw_breakpoint_restore();
 
-        set_bit(KVM_REQ_KICK, &vcpu->requests);
+        atomic_set(&vcpu->guest_mode, 0);
+        smp_wmb();
         local_irq_enable();
 
         ++vcpu->stat.exits;
@@ -5580,7 +5583,7 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
 
         me = get_cpu();
         if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
-                if (!test_and_set_bit(KVM_REQ_KICK, &vcpu->requests))
+                if (atomic_xchg(&vcpu->guest_mode, 0))
                         smp_send_reschedule(cpu);
         put_cpu();
 }
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index a0e019769f5d..2c62319727ef 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -81,6 +81,7 @@ struct kvm_vcpu {
         int vcpu_id;
         struct mutex mutex;
         int   cpu;
+        atomic_t guest_mode;
         struct kvm_run *run;
         unsigned long requests;
         unsigned long guest_debug;