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[PULL 12/20] virtio: use virtio accessor to access packed event

From: Michael S. Tsirkin
Subject: [PULL 12/20] virtio: use virtio accessor to access packed event
Date: Mon, 15 Nov 2021 11:38:34 -0500

From: Jason Wang <jasowang@redhat.com>

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

This patch fixes this by switching to use
virito_{stw|lduw}_phys_cached() to make sure the access is atomic.

Fixes: 683f7665679c1 ("virtio: event suppression support for packed ring")
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20211111063854.29060-2-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>
 hw/virtio/virtio.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 939bcbfeb9..ea7c079fb0 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -247,13 +247,10 @@ static void vring_packed_event_read(VirtIODevice *vdev,
     hwaddr off_off = offsetof(VRingPackedDescEvent, off_wrap);
     hwaddr off_flags = offsetof(VRingPackedDescEvent, flags);
-    address_space_read_cached(cache, off_flags, &e->flags,
-                              sizeof(e->flags));
+    e->flags = virtio_lduw_phys_cached(vdev, cache, off_flags);
     /* Make sure flags is seen before off_wrap */
-    address_space_read_cached(cache, off_off, &e->off_wrap,
-                              sizeof(e->off_wrap));
-    virtio_tswap16s(vdev, &e->off_wrap);
+    e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off);
     virtio_tswap16s(vdev, &e->flags);
@@ -263,8 +260,7 @@ static void vring_packed_off_wrap_write(VirtIODevice *vdev,
     hwaddr off = offsetof(VRingPackedDescEvent, off_wrap);
-    virtio_tswap16s(vdev, &off_wrap);
-    address_space_write_cached(cache, off, &off_wrap, sizeof(off_wrap));
+    virtio_stw_phys_cached(vdev, cache, off, off_wrap);
     address_space_cache_invalidate(cache, off, sizeof(off_wrap));
@@ -273,8 +269,7 @@ static void vring_packed_flags_write(VirtIODevice *vdev,
     hwaddr off = offsetof(VRingPackedDescEvent, flags);
-    virtio_tswap16s(vdev, &flags);
-    address_space_write_cached(cache, off, &flags, sizeof(flags));
+    virtio_stw_phys_cached(vdev, cache, off, flags);
     address_space_cache_invalidate(cache, off, sizeof(flags));

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