Re: [PATCH net] virtio-net: fix incorrect flags recording in big mode

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Sep 19, 2025 at 10:11:55AM +0800, Jason Wang wrote:
> On Fri, Sep 19, 2025 at 9:35 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > The purpose of commit 703eec1b2422 ("virtio_net: fixing XDP for fully
> > checksummed packets handling") is to record the flags in advance, as
> > their value may be overwritten in the XDP case. However, the flags
> > recorded under big mode are incorrect, because in big mode, the passed
> > buf does not point to the rx buffer, but rather to the page of the
> > submitted buffer. This commit fixes this issue.
> >
> > For the small mode, the commit c11a49d58ad2 ("virtio_net: Fix mismatched
> > buf address when unmapping for small packets") fixed it.
> >
> > Fixes: 703eec1b2422 ("virtio_net: fixing XDP for fully checksummed packets handling")
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  drivers/net/virtio_net.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 975bdc5dab84..6e6e74390955 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2630,13 +2630,19 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> >          */
> >         flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
> >
> > -       if (vi->mergeable_rx_bufs)
> > +       if (vi->mergeable_rx_bufs) {
> >                 skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
> >                                         stats);
> > -       else if (vi->big_packets)
> > +       } else if (vi->big_packets) {
> > +               void *p;
> > +
> > +               p = page_address((struct page *)buf);
> > +               flags = ((struct virtio_net_common_hdr *)p)->hdr.flags;
> > +
> 
> Patch looks good but a I have a nit:
> 
> It looks better to move this above?
> 
> if (vi->big_packets) {
>                void *p = page_address((struct page *)buf);
>                flags = ((struct virtio_net_common_hdr *)p)->hdr.flags;
> } else
>                flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
> 
> To avoid twice the calculations and reuse the comment.


I think duplicating a bit of code is better than branching twice.
So I would move the flags assignment down instead.


> >                 skb = receive_big(dev, vi, rq, buf, len, stats);
> > -       else
> > +       } else {
> >                 skb = receive_small(dev, vi, rq, buf, ctx, len, xdp_xmit, stats);
> > +       }
> >
> >         if (unlikely(!skb))
> >                 return;
> > --
> > 2.32.0.3.g01195cf9f
> >
> 
> Thanks