[PATCH 36/47] virtio: use virtio accessor to access packed descriptor flags

[Michael Roth]

From: Jason Wang <jasowang@redhat.com>

We used to access packed descriptor flags via
address_space_{write|read}_cached(). When we hit the cache, memcpy()
is used which is not an atomic operation which may lead a wrong value
is read or wrote.

So this patch switches to use virito_{stw|lduw}_phys_cached() to make
sure the aceess is atomic.

Fixes: 86044b24e865f ("virtio: basic packed virtqueue support")
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20211111063854.29060-1-jasowang@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit f463e761a41ee71e59892121e1c74d9c25c985d2)
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 hw/virtio/virtio.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 874377f37a..2dbccb6b3f 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -509,11 +509,9 @@ static void vring_packed_desc_read_flags(VirtIODevice 
*vdev,
                                          MemoryRegionCache *cache,
                                          int i)
 {
-    address_space_read_cached(cache,
-                              i * sizeof(VRingPackedDesc) +
-                              offsetof(VRingPackedDesc, flags),
-                              flags, sizeof(*flags));
-    virtio_tswap16s(vdev, flags);
+    hwaddr off = i * sizeof(VRingPackedDesc) + offsetof(VRingPackedDesc, 
flags);
+
+    *flags = virtio_lduw_phys_cached(vdev, cache, off);
 }
 
 static void vring_packed_desc_read(VirtIODevice *vdev,
@@ -566,8 +564,7 @@ static void vring_packed_desc_write_flags(VirtIODevice 
*vdev,
 {
     hwaddr off = i * sizeof(VRingPackedDesc) + offsetof(VRingPackedDesc, 
flags);
 
-    virtio_tswap16s(vdev, &desc->flags);
-    address_space_write_cached(cache, off, &desc->flags, sizeof(desc->flags));
+    virtio_stw_phys_cached(vdev, cache, off, desc->flags);
     address_space_cache_invalidate(cache, off, sizeof(desc->flags));
 }
 
-- 
2.25.1