linux-kernel - Re: [PATCH] virtio_net: Support RX hash XDP hint

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Date: Wed, 24 Jan 2024 16:03:51 +0800
From: Liang Chen <liangchen.linux@...il.com>
To: Jason Wang <jasowang@...hat.com>
Cc: mst@...hat.com, virtualization@...ts.linux-foundation.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_net: Support RX hash XDP hint

On Wed, Jan 24, 2024 at 10:12 AM Jason Wang <jasowang@...hat.com> wrote:
>
> On Mon, Jan 22, 2024 at 6:23 PM Liang Chen <liangchen.linux@...ilcom> wrote:
> >
> > The RSS hash report is a feature that's part of the virtio specification.
> > Currently, virtio backends like qemu, vdpa (mlx5), and potentially vhost
> > (still a work in progress as per [1]) support this feature. While the
> > capability to obtain the RSS hash has been enabled in the normal path,
> > it's currently missing in the XDP path. Therefore, we are introducing XDP
> > hints through kfuncs to allow XDP programs to access the RSS hash.
> >
> > Signed-off-by: Liang Chen <liangchen.linux@...il.com>
> > ---
> >  drivers/net/virtio_net.c | 56 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 56 insertions(+)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index d7ce4a1011ea..1463a4709e3c 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -4579,6 +4579,60 @@ static void virtnet_set_big_packets(struct virtnet_info *vi, const int mtu)
> >         }
> >  }
> >
> > +static int virtnet_xdp_rx_hash(const struct xdp_md *_ctx, u32 *hash,
> > +                          enum xdp_rss_hash_type *rss_type)
> > +{
> > +       const struct xdp_buff *xdp = (void *)_ctx;
> > +       struct virtio_net_hdr_v1_hash *hdr_hash;
> > +       struct virtnet_info *vi;
> > +
> > +       if (!(xdp->rxq->dev->features & NETIF_F_RXHASH))
> > +               return -ENODATA;
> > +
> > +       vi = netdev_priv(xdp->rxq->dev);
> > +       hdr_hash = (struct virtio_net_hdr_v1_hash *)(xdp->data - vi->hdr_len);
>
> Is there a guarantee that the hdr is not modified?
>

We cannot guarantee the hdr is not modified by the XDP prog, So the
idea is to save it in a wrapper structure before running the xdp prog.
Patch is coming soon.

Thanks,
Liang

> Thanks
>