Re: [PATCH] vhost/net: Set num_buffers for virtio 1.0

[Jason Wang]

On Wed, Nov 6, 2024 at 4:54 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Sun, Sep 15, 2024 at 10:35:53AM +0900, Akihiko Odaki wrote:
> > The specification says the device MUST set num_buffers to 1 if
> > VIRTIO_NET_F_MRG_RXBUF has not been negotiated.
> >
> > Fixes: 41e3e42108bc ("vhost/net: enable virtio 1.0")
> > Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
>
> True, this is out of spec. But, qemu is also out of spec :(
>
> Given how many years this was out there, I wonder whether
> we should just fix the spec, instead of changing now.
>
> Jason, what's your take?

Fixing the spec (if you mean release the requirement) seems to be less risky.

Thanks

>
>
> > ---
> >  drivers/vhost/net.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index f16279351db5..d4d97fa9cc8f 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -1107,6 +1107,7 @@ static void handle_rx(struct vhost_net *net)
> >       size_t vhost_hlen, sock_hlen;
> >       size_t vhost_len, sock_len;
> >       bool busyloop_intr = false;
> > +     bool set_num_buffers;
> >       struct socket *sock;
> >       struct iov_iter fixup;
> >       __virtio16 num_buffers;
> > @@ -1129,6 +1130,8 @@ static void handle_rx(struct vhost_net *net)
> >       vq_log = unlikely(vhost_has_feature(vq, VHOST_F_LOG_ALL)) ?
> >               vq->log : NULL;
> >       mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
> > +     set_num_buffers = mergeable ||
> > +                       vhost_has_feature(vq, VIRTIO_F_VERSION_1);
> >
> >       do {
> >               sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
> > @@ -1205,7 +1208,7 @@ static void handle_rx(struct vhost_net *net)
> >               /* TODO: Should check and handle checksum. */
> >
> >               num_buffers = cpu_to_vhost16(vq, headcount);
> > -             if (likely(mergeable) &&
> > +             if (likely(set_num_buffers) &&
> >                   copy_to_iter(&num_buffers, sizeof num_buffers,
> >                                &fixup) != sizeof num_buffers) {
> >                       vq_err(vq, "Failed num_buffers write");
> >
> > ---
> > base-commit: 46a0057a5853cbdb58211c19e89ba7777dc6fd50
> > change-id: 20240908-v1-90fc83ff8b09
> >
> > Best regards,
> > --
> > Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
>