virtio: order index/descriptor reads

virtio has the equivalent of:

	if (vq->last_avail_index != vring_avail_idx(vq)) {
		read descriptor head at vq->last_avail_index;
	}

In theory, processor can reorder descriptor head
read to happen speculatively before the index read.
this would trigger the following race:

	host descriptor head read <- reads invalid head from ring
		guest writes valid descriptor head
		guest writes avail index
	host avail index read <- observes valid index

as a result host will use an invalid head value.
This was not observed in the field by me but after
the experience with the previous two races
I think it is prudent to address this theoretical race condition.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio.c

@@ -287,6 +287,11 @@ static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
                      idx, vring_avail_idx(vq));
         exit(1);
     }
+    /* On success, callers read a descriptor at vq->last_avail_idx.
+     * Make sure descriptor read does not bypass avail index read. */
+    if (num_heads) {
+        smp_rmb();
+    }
 
     return num_heads;
 }

qemu-barrier.h

@@ -7,12 +7,13 @@
 #if defined(__i386__)
 
 /*
- * Because of the strongly ordered x86 storage model, wmb() is a nop
+ * Because of the strongly ordered x86 storage model, wmb() and rmb() are nops
  * on x86(well, a compiler barrier only).  Well, at least as long as
  * qemu doesn't do accesses to write-combining memory or non-temporal
  * load/stores from C code.
  */
 #define smp_wmb()   barrier()
+#define smp_rmb()   barrier()
 /*
  * We use GCC builtin if it's available, as that can use
  * mfence on 32 bit as well, e.g. if built with -march=pentium-m.
@@ -27,6 +28,7 @@
 #elif defined(__x86_64__)
 
 #define smp_wmb()   barrier()
+#define smp_rmb()   barrier()
 #define smp_mb() asm volatile("mfence" ::: "memory")
 
 #elif defined(_ARCH_PPC)
@@ -37,6 +39,13 @@
  * each other
  */
 #define smp_wmb()   asm volatile("eieio" ::: "memory")
+
+#if defined(__powerpc64__)
+#define smp_rmb()   asm volatile("lwsync" ::: "memory")
+#else
+#define smp_rmb()   asm volatile("sync" ::: "memory")
+#endif
+
 #define smp_mb()   asm volatile("sync" ::: "memory")
 
 #else
@@ -45,10 +54,11 @@
  * For (host) platforms we don't have explicit barrier definitions
  * for, we use the gcc __sync_synchronize() primitive to generate a
  * full barrier.  This should be safe on all platforms, though it may
- * be overkill for wmb().
+ * be overkill for wmb() and rmb().
  */
 #define smp_wmb()   __sync_synchronize()
 #define smp_mb()   __sync_synchronize()
+#define smp_rmb()   __sync_synchronize()
 
 #endif