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>

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;