On Thu, May 9, 2024 at 4:20 AM Wafer <wa...@jaguarmicro.com> wrote:
>
>
>
> 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.
>
Ok now after reading the driver code again I see how I missed that.
Sorry for the noise!
> 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
> > > >
>
