On Thu, May, 2024 at 2:21 AM Michael S. Tsirkin <m...@redhat.com> wrote:
>
> On Wed, May 08, 2024 at 02:56:11PM +0200, Eugenio Perez Martin wrote:
> > 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?
> >
The linux virtio_ring driver set the buffer id into all the descriptors of the
chain.
So Bad things can't happen, with this patch, the Linux VirtIO driver can work
properly.
I have tested it.
> > If it breaks I guess this requires more thinking. I didn't check DPDK,
> > neither as driver nor as vhost-user device.
> >
> > Thanks!
>
> I think that if the driver is out of spec we should for starters fix it ASAP.
The linux driver is within spec.
>
> > > 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
> > >
