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

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Message-ID: <CACGkMEsNsQ_XWTvdjaCEdo8sYaLew24zU1UUCJrokM-Koxj4fw@mail.gmail.com>
Date:   Thu, 25 Nov 2021 15:15:33 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     mst <mst@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] virtio-net: enable big mode correctly

On Thu, Nov 25, 2021 at 3:09 PM Eli Cohen <elic@...dia.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. We can do optimization on top.
> >
> > Reported-by: Eli Cohen <elic@...dia.com>
> > Cc: Eli Cohen <elic@...dia.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.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 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).

Thanks

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