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

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Date:   Thu, 25 Nov 2021 15:11:58 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eli Cohen <elic@...dia.com>
Subject: Re: [PATCH net] virtio-net: enable big mode correctly

On Thu, Nov 25, 2021 at 3:00 PM Michael S. Tsirkin <mst@...hat.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.

> 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

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