[PATCH 4/5] KVM: add __kvm_request_needs_mb

["Radim Krčmář" <rkrcmar@redhat.com>]

A macro to optimize requests that do not need a memory barrier because
they have no dependencies.  An architecture can implement a function
that says which requests do not need memory barriers when handling them.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
 include/linux/kvm_host.h | 41 +++++++++++++++++++++++++++++++++++++----
 virt/kvm/kvm_main.c      |  3 ++-
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d899473859d3..2cc438685af8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1097,8 +1097,8 @@ static inline int kvm_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
  *  2) remote request with no data (= kick)
  *  3) remote request with data (= kick + mb)
  *
- * TODO: the API is inconsistent -- a request doesn't call kvm_vcpu_kick(), but
- * forces smp_wmb() for all requests.
+ * TODO: the API does not distinguish local and remote requests -- remote
+ * should contain kvm_vcpu_kick().
  */
 
 static inline void __kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
@@ -1106,6 +1106,37 @@ static inline void __kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
 	set_bit(req, &vcpu->requests);
 }
 
+/*
+ * __kvm_request_needs_mb is used to improve performance, so it should have no
+ * runtime overhead.
+ */
+static inline bool __kvm_request_needs_mb(int req)
+{
+	/*
+	 * This barrier lets callers avoid the following pattern:
+	 *   if (__kvm_request_needs_mb(req))
+	 *      ...
+	 *   else
+	 *      barrier();
+	 */
+	barrier();
+
+	if (!__builtin_constant_p(req))
+		return true;
+
+#ifdef kvm_arch_request_needs_mb
+	/*
+	 * GCC optimizes pure kvm_arch_request_needs_mb() with a constant input
+	 * into a contant, but __builtin_constant_p() is not so clever, so we
+	 * cannot ensure that with:
+	 * BUILD_BUG_ON(!__builtin_constant_p(kvm_arch_request_needs_mb(req)));
+	 */
+	return kvm_arch_request_needs_mb(req);
+#else
+	return true;
+#endif
+}
+
 static inline void kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
 {
 	/*
@@ -1113,7 +1144,8 @@ static inline void kvm_request_set(unsigned req, struct kvm_vcpu *vcpu)
 	 * kvm_request_test_and_clear's caller.
 	 * Paired with the smp_mb__after_atomic in kvm_request_test_and_clear.
 	 */
-	smp_wmb();
+	if (__kvm_request_needs_mb(req))
+		smp_wmb();
 	__kvm_request_set(req, vcpu);
 }
 
@@ -1137,7 +1169,8 @@ static inline bool kvm_request_test_and_clear(unsigned req, struct kvm_vcpu *vcp
 		 * kvm_request_test_and_clear's caller.
 		 * Paired with the smp_wmb in kvm_request_set.
 		 */
-		smp_mb__after_atomic();
+		if (__kvm_request_needs_mb(req))
+			smp_mb__after_atomic();
 		return true;
 	} else {
 		return false;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2250920ec965..ced3e4cb1df0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -179,7 +179,8 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
 	me = get_cpu();
 
 	/* Paired with the smp_mb__after_atomic in kvm_request_test_and_clear. */
-	smp_wmb();
+	if (__kvm_request_needs_mb(req))
+		smp_wmb();
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		__kvm_request_set(req, vcpu);
-- 
2.11.1