On Mon, Apr 22, 2024 at 3:41 AM Wafer <wa...@jaguarmicro.com> wrote:
>
> 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.
>
This is actually trickier. While it is true the standard mandates it,
both linux virtio_ring driver and QEMU trusts the ID will be the first
descriptor of the chain. Does merging this change in QEMU without
merging the corresponding one in the linux kernel break things? Or am
I missing something?
If it breaks I guess this requires more thinking. I didn't check DPDK,
neither as driver nor as vhost-user device.
Thanks!
> Fixes: 86044b24e8 ("virtio: basic packed virtqueue support")
>
> Signed-off-by: Wafer <wa...@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
>
