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SubjectRe: [PATCH net] virtio-net: enable big mode correctly
On Thu, Nov 25, 2021 at 3:09 PM Eli Cohen <elic@nvidia.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@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 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
> >
>

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