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

[Michael S. Tsirkin]

On Thu, Nov 25, 2021 at 03:26:22PM +0800, Jason Wang wrote:
> On Thu, Nov 25, 2021 at 3:20 PM Eli Cohen <elic@xxxxxxxxxx> wrote:
> >
> > On Thu, Nov 25, 2021 at 03:15:33PM +0800, Jason Wang wrote:
> > > On Thu, Nov 25, 2021 at 3:09 PM Eli Cohen <elic@xxxxxxxxxx> 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. We can do optimization on top.
> > > > >
> > > > > Reported-by: Eli Cohen <elic@xxxxxxxxxx>
> > > > > Cc: Eli Cohen <elic@xxxxxxxxxx>
> > > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > > > ---
> > > > >  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 VIRTIO_NET_F_MTU is provided, then dev->max_mtu is going to equal
> > > > ETH_DATA_LEN (will be set in ether_setup()) so I don't think it will set
> > > > big_packets to true.
> > >
> > > I may miss something, the dev->max_mtu is just assigned to the mtu
> > > value read from the config space in the code block above  (inside the
> > > feature check of VIRTIO_NET_F_MTU).
> >
> > Sorry, I meant "If VIRTIO_NET_F_MTU is ***NOT*** provided". In that case
> > dev->max_mtu eauals ETH_DATA_LEN so you won't set vi->big_packets to
> > true.
> 
> I see but in this case, the above assignment:
> 
>         /* MTU range: 68 - 65535 */
>         dev->min_mtu = MIN_MTU;
>         dev->max_mtu = MAX_MTU;
> 
> happens after alloc_etherdev_mq() which calls ether_setup(), so we are
> probably fine here.
> 
> Thanks

Actually the issue with VIRTIO_NET_F_MTU is that devices might be
tempted to expose 9k here simply because they support jumbo frames,
if they also don't support mergeable buffers this will force big
packet mode.


> >
> > >
> > > Thanks
> > >
> > > >
> > > >
> > > > >       if (vi->any_header_sg)
> > > > >               dev->needed_headroom = vi->hdr_len;
> > > > >
> > > > > --
> > > > > 2.25.1
> > > > >
> > > >
> > >
> >