Re: [PATCH net-next v5] virtio_net: Support RX hash XDP hint

[Paolo Abeni]

On Wed, 2024-02-07 at 10:54 +0800, Liang Chen wrote:
> On Tue, Feb 6, 2024 at 6:44 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
> > 
> > On Sat, 2024-02-03 at 10:56 +0800, Liang Chen wrote:
> > > On Sat, Feb 3, 2024 at 12:20 AM Jesper Dangaard Brouer <hawk@xxxxxxxxxx> wrote:
> > > > On 02/02/2024 13.11, Liang Chen wrote:
> > [...]
> > > > > @@ -1033,6 +1039,16 @@ static void put_xdp_frags(struct xdp_buff *xdp)
> > > > >       }
> > > > >   }
> > > > > 
> > > > > +static void virtnet_xdp_save_rx_hash(struct virtnet_xdp_buff *virtnet_xdp,
> > > > > +                                  struct net_device *dev,
> > > > > +                                  struct virtio_net_hdr_v1_hash *hdr_hash)
> > > > > +{
> > > > > +     if (dev->features & NETIF_F_RXHASH) {
> > > > > +             virtnet_xdp->hash_value = hdr_hash->hash_value;
> > > > > +             virtnet_xdp->hash_report = hdr_hash->hash_report;
> > > > > +     }
> > > > > +}
> > > > > +
> > > > 
> > > > Would it be possible to store a pointer to hdr_hash in virtnet_xdp_buff,
> > > > with the purpose of delaying extracting this, until and only if XDP
> > > > bpf_prog calls the kfunc?
> > > > 
> > > 
> > > That seems to be the way v1 works,
> > > https://lore.kernel.org/all/20240122102256.261374-1-liangchen.linux@xxxxxxxxx/
> > > . But it was pointed out that the inline header may be overwritten by
> > > the xdp prog, so the hash is copied out to maintain its integrity.
> > 
> > Why? isn't XDP supposed to get write access only to the pkt
> > contents/buffer?
> > 
> 
> Normally, an XDP program accesses only the packet data. However,
> there's also an XDP RX Metadata area, referenced by the data_meta
> pointer. This pointer can be adjusted with bpf_xdp_adjust_meta to
> point somewhere ahead of the data buffer, thereby granting the XDP
> program access to the virtio header located immediately before the

AFAICS bpf_xdp_adjust_meta() does not allow moving the meta_data before
xdp->data_hard_start:

https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4210

and virtio net set such field after the virtio_net_hdr:

https://elixir.bootlin.com/linux/latest/source/drivers/net/virtio_net.c#L1218
https://elixir.bootlin.com/linux/latest/source/drivers/net/virtio_net.c#L1420

I don't see how the virtio hdr could be touched? Possibly even more
important: if such thing is possible, I think is should be somewhat
denied (for the same reason an H/W nic should prevent XDP from
modifying its own buffer descriptor).

Cheers,

Paolo