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From: Eugenio Perez Martin <eperezma@redhat.com>
To: Wafer <wafer@jaguarmicro.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	 "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	Angus Chen <angus.chen@jaguarmicro.com>,
	Lei Yang <leiyang@redhat.com>
Subject: Re: [PATCH] hw/virtio: Fix obtain the buffer id from the last descriptor
Date: Thu, 9 May 2024 07:44:45 +0200	[thread overview]
Message-ID: <CAJaqyWeoFr=kupOZmOpAH2qu3mSZSntrYY3F0ty8wFCvpa3okA@mail.gmail.com> (raw)
In-Reply-To: <PUZPR06MB47138D7EAFA0BB70931D4832A2E62@PUZPR06MB4713.apcprd06.prod.outlook.com>

On Thu, May 9, 2024 at 4:20 AM Wafer <wafer@jaguarmicro.com> wrote:
>
>
>
> On Thu, May, 2024 at 2:21 AM Michael S. Tsirkin <mst@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 <wafer@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 <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
> > > >
>



      reply	other threads:[~2024-05-09  5:46 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-22  1:40 [PATCH] hw/virtio: Fix obtain the buffer id from the last descriptor Wafer
2024-05-08  4:00 ` Jason Wang
2024-05-09  4:32   ` Wafer
2024-05-09  5:45     ` Eugenio Perez Martin
2024-05-08 12:56 ` Eugenio Perez Martin
2024-05-08 18:21   ` Michael S. Tsirkin
2024-05-09  2:20     ` Wafer
2024-05-09  5:44       ` Eugenio Perez Martin [this message]

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