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

[Michael S. Tsirkin]

On Thu, Nov 25, 2021 at 03:20:07PM +0800, Jason Wang wrote:
> On Thu, Nov 25, 2021 at 3:15 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> > 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@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.
> > > >
> > > > 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?
> 
> Yes, or fix the issue to let large packets on RX work (e.g as the TODO
> said, size the buffer: for <=4K mtu continue to work as
> add_recvbuf_small(), for >= 4K switch to use big).

Right. The difficulty is with changing modes, current code isn't
designed for it.

> > Maybe ... it will prevent sending large packets which did work ...
> 
> Yes, but it's strange to allow TX but not RX
> 
> > I'd tread carefully here, and I don't think this kind of thing is net
> > material.
> 
> I agree consider it can't be fixed easily.
> 
> Thanks
> 
> >
> > > >
> > > > > 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 (vi->any_header_sg)
> > > > >               dev->needed_headroom = vi->hdr_len;
> > > > >
> > > > > --
> > > > > 2.25.1
> > > >
> >