Re: [PATCH net] virtio-net: enable big mode correctly

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 25, 2021 at 03:11:58PM +0800, Jason Wang wrote:
> On Thu, Nov 25, 2021 at 3:00 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Nov 25, 2021 at 02:05:47PM +0800, Jason Wang wrote:
> > > When VIRTIO_NET_F_MTU feature is not negotiated, we assume a very
> > > large max_mtu. In this case, using small packet mode is not correct
> > > since it may breaks the networking when MTU is grater than
> > > ETH_DATA_LEN.
> > >
> > > To have a quick fix, simply enable the big packet mode when
> > > VIRTIO_NET_F_MTU is not negotiated.
> >
> > This will slow down dpdk hosts which disable mergeable buffers
> > and send standard MTU sized packets.
> >
> > > We can do optimization on top.
> >
> > I don't think it works like this, increasing mtu
> > from guest >4k never worked,
> 
> Looking at add_recvbuf_small() it's actually GOOD_PACKET_LEN if I was not wrong.

OK, even more so then.

> > we can't regress everyone's
> > performance with a promise to maybe sometime bring it back.
> 
> So consider it never work before I wonder if we can assume a 1500 as
> max_mtu value instead of simply using MAX_MTU?
> 
> Thanks

You want to block guests from setting MTU to a value >GOOD_PACKET_LEN?
Maybe ... it will prevent sending large packets which did work ...
I'd tread carefully here, and I don't think this kind of thing is net
material.

> >
> > > Reported-by: Eli Cohen <elic@nvidia.com>
> > > Cc: Eli Cohen <elic@nvidia.com>
> > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > >
> > > ---
> > >  drivers/net/virtio_net.c | 7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 7c43bfc1ce44..83ae3ef5eb11 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -3200,11 +3200,12 @@ static int virtnet_probe(struct virtio_device *vdev)
> > >               dev->mtu = mtu;
> > >               dev->max_mtu = mtu;
> > >
> > > -             /* TODO: size buffers correctly in this case. */
> > > -             if (dev->mtu > ETH_DATA_LEN)
> > > -                     vi->big_packets = true;
> > >       }
> > >
> > > +     /* TODO: size buffers correctly in this case. */
> > > +     if (dev->max_mtu > ETH_DATA_LEN)
> > > +             vi->big_packets = true;
> > > +
> > >       if (vi->any_header_sg)
> > >               dev->needed_headroom = vi->hdr_len;
> > >
> > > --
> > > 2.25.1
> >