cmm: provide lightweight smp_rmb/smp_wmb on PPC

lwsync orders loads in cacheable memory with respect to other loads,
and stores in cacheable memory with respect to other stores.  Use it
to implement smp_rmb/smp_wmb.

The heavy-weight sync is still used for the "full" rmb/wmb operations,
as well as for smp_mb.

[ Edit by Mathieu Desnoyers: rephrased the comments around the memory
  barriers. ]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu/arch/ppc.h b/urcu/arch/ppc.h
index a03d688837344f0bb2d438c7161d43dcb208a5ef..048b217392cc7cd471647625ca1dd128e6627e00 100644 (file)
--- a/urcu/arch/ppc.h
+++ b/urcu/arch/ppc.h
@@ -32,7 +32,24 @@ extern "C" {
 /* Include size of POWER5+ L3 cache lines: 256 bytes */
 #define CAA_CACHE_LINE_SIZE    256
 
-#define cmm_mb()    asm volatile("sync":::"memory")
+/*
+ * Use sync for all cmm_mb/rmb/wmb barriers because lwsync does not
+ * preserve ordering of cacheable vs. non-cacheable accesses, so it
+ * should not be used to order with respect to MMIO operations.  An
+ * eieio+lwsync pair is also not enough for cmm_rmb, because it will
+ * order cacheable and non-cacheable memory operations separately---i.e.
+ * not the latter against the former.
+ */
+#define cmm_mb()         asm volatile("sync":::"memory")
+
+/*
+ * lwsync orders loads in cacheable memory with respect to other loads,
+ * and stores in cacheable memory with respect to other stores.
+ * Therefore, use it for barriers ordering accesses to cacheable memory
+ * only.
+ */
+#define cmm_smp_rmb()    asm volatile("lwsync":::"memory")
+#define cmm_smp_wmb()    asm volatile("lwsync":::"memory")
 
 #define mftbl()                                                \
        ({                                              \