[Qemu-devel] [PATCH-RFC 3/3] virtio: add missing barriers

[Michael S. Tsirkin]

Add missing memory barriers in virtio:
- before checking interrupt enabled bit, make sure
  data has been written to memory
- make sure ring index has been read before reading ring data

Signed-off-by: Michael S. Tsirkin <address@hidden>
---
 hw/virtio.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/hw/virtio.c b/hw/virtio.c
index 9f020cf..bbd7b51 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -119,8 +119,12 @@ static inline uint16_t vring_avail_flags(VirtQueue *vq)
 static inline uint16_t vring_avail_idx(VirtQueue *vq)
 {
     target_phys_addr_t pa;
+    uint16_t idx;
     pa = vq->vring.avail + offsetof(VRingAvail, idx);
-    return lduw_phys(pa);
+    idx = lduw_phys(pa);
+    /* Make sure data read happens after index read */
+    rmb();
+    return idx;
 }
 
 static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
@@ -588,6 +592,8 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int 
queue_size,
 
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
+    /* Make sure used ring is updated before we check NO_INTERRUPT. */
+    mb();
     /* Always notify when queue is empty (when feature acknowledge) */
     if ((vring_avail_flags(vq) & VRING_AVAIL_F_NO_INTERRUPT) &&
         (!(vdev->features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY)) ||
-- 
1.6.5.2.143.g8cc62