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

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>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20240510072753.26158-2-wafer@jaguarmicro.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 28cd406e1684c870121bd9a00c8b7774a81c0f97..3678ec2f88b5cacc84e393d009a2d760624ef4f6 100644 (file)
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1745,6 +1745,11 @@ static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz)
                                              &indirect_desc_cache);
     } while (rc == VIRTQUEUE_READ_DESC_MORE);
 
+    if (desc_cache != &indirect_desc_cache) {
+        /* Buffer ID is included in the last descriptor in the list. */
+        id = desc.id;
+    }
+
     /* Now copy what we have collected and mapped */
     elem = virtqueue_alloc_element(sz, out_num, in_num);
     for (i = 0; i < out_num; i++) {