hw/virtio: Fix obtain the buffer id from the last descriptor

[PATCH] hw/virtio: Fix obtain the buffer id from the last descriptor

Wafer posted 1 patch 1 week, 5 days ago

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git fetch https://github.com/patchew-project/qemu tags/patchew/20240422014041.5706-2-wafer@jaguarmicro.com

Maintainers: "Michael S. Tsirkin" <mst@redhat.com>

hw/virtio/virtio.c | 5 +++++
1 file changed, 5 insertions(+)

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[PATCH] hw/virtio: Fix obtain the buffer id from the last descriptor

Posted by Wafer 1 week, 5 days ago

The virtio-1.3 specification
<https://docs.oasis-open.org/virtio/virtio/v1.3/virtio-v1.3.html> writes:
2.8.6 Next Flag: Descriptor Chaining
      Buffer ID is included in the last descriptor in the list.

If the feature (_F_INDIRECT_DESC) has been negotiated, install only
one descriptor in the virtqueue.
Therefor the buffer id should be obtained from the first descriptor.

In descriptor chaining scenarios, the buffer id should be obtained
from the last descriptor.

Fixes: 86044b24e8 ("virtio: basic packed virtqueue support")

Signed-off-by: Wafer <wafer@jaguarmicro.com>
---
 hw/virtio/virtio.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 871674f9be..f65d4b4161 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1739,6 +1739,11 @@ static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz)
             goto err_undo_map;
         }
 
+        if (desc_cache != &indirect_desc_cache) {
+            /* Buffer ID is included in the last descriptor in the list. */
+            id = desc.id;
+        }
+
         rc = virtqueue_packed_read_next_desc(vq, &desc, desc_cache, max, &i,
                                              desc_cache ==
                                              &indirect_desc_cache);
-- 
2.27.0