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Date: Mon, 26 Feb 2024 12:42:15 -0800
From: John Fastabend <john.fastabend@gmail.com>
To: Jason Wang <jasowang@redhat.com>, 
 Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Paolo Abeni <pabeni@redhat.com>, 
 Jesper Dangaard Brouer <hawk@kernel.org>, 
 mst@redhat.com, 
 hengqi@linux.alibaba.com, 
 davem@davemloft.net, 
 edumazet@google.com, 
 kuba@kernel.org, 
 netdev@vger.kernel.org, 
 virtualization@lists.linux.dev, 
 linux-kernel@vger.kernel.org, 
 bpf@vger.kernel.org, 
 john.fastabend@gmail.com, 
 daniel@iogearbox.net, 
 ast@kernel.org, 
 Liang Chen <liangchen.linux@gmail.com>
Message-ID: <65dcf7a775437_20e0a2087f@john.notmuch>
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Subject: Re: [PATCH net-next v5] virtio_net: Support RX hash XDP hint
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Jason Wang wrote:
> On Fri, Feb 23, 2024 at 9:42=E2=80=AFAM Xuan Zhuo <xuanzhuo@linux.aliba=
ba.com> wrote:
> >
> > On Fri, 09 Feb 2024 13:57:25 +0100, Paolo Abeni <pabeni@redhat.com> w=
rote:
> > > On Fri, 2024-02-09 at 18:39 +0800, Liang Chen wrote:
> > > > On Wed, Feb 7, 2024 at 10:27=E2=80=AFPM Paolo Abeni <pabeni@redha=
t.com> wrote:
> > > > >
> > > > > On Wed, 2024-02-07 at 10:54 +0800, Liang Chen wrote:
> > > > > > On Tue, Feb 6, 2024 at 6:44=E2=80=AFPM Paolo Abeni <pabeni@re=
dhat.com> wrote:
> > > > > > >
> > > > > > > On Sat, 2024-02-03 at 10:56 +0800, Liang Chen wrote:
> > > > > > > > On Sat, Feb 3, 2024 at 12:20=E2=80=AFAM Jesper Dangaard B=
rouer <hawk@kernel.org> wrote:
> > > > > > > > > On 02/02/2024 13.11, Liang Chen wrote:
> > > > > > > [...]
> > > > > > > > > > @@ -1033,6 +1039,16 @@ static void put_xdp_frags(stru=
ct 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 =3D hdr_hash->h=
ash_value;
> > > > > > > > > > +             virtnet_xdp->hash_report =3D 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-liang=
chen.linux@gmail.com/
> > > > > > > > . But it was pointed out that the inline header may be ov=
erwritten by
> > > > > > > > the xdp prog, so the hash is copied out to maintain its i=
ntegrity.
> > > > > > >
> > > > > > > Why? isn't XDP supposed to get write access only to the pkt=

> > > > > > > contents/buffer?
> > > > > > >
> > > > > >
> > > > > > Normally, an XDP program accesses only the packet data. Howev=
er,
> > > > > > there's also an XDP RX Metadata area, referenced by the data_=
meta
> > > > > > pointer. This pointer can be adjusted with bpf_xdp_adjust_met=
a to
> > > > > > point somewhere ahead of the data buffer, thereby granting th=
e XDP
> > > > > > program access to the virtio header located immediately befor=
e the
> > > > >
> > > > > AFAICS bpf_xdp_adjust_meta() does not allow moving the meta_dat=
a 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/virt=
io_net.c#L1218
> > > > > https://elixir.bootlin.com/linux/latest/source/drivers/net/virt=
io_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 some=
what
> > > > > denied (for the same reason an H/W nic should prevent XDP from
> > > > > modifying its own buffer descriptor).
> > > >
> > > > Thank you for highlighting this concern. The header layout differ=
s
> > > > slightly between small and mergeable mode. Taking 'mergeable mode=
' as
> > > > an example, after calling xdp_prepare_buff the layout of xdp_buff=

> > > > would be as depicted in the diagram below,
> > > >
> > > >                       buf
> > > >                        |
> > > >                        v
> > > >         +--------------+--------------+-------------+
> > > >         | xdp headroom | virtio header| packet      |
> > > >         | (256 bytes)  | (20 bytes)   | content     |
> > > >         +--------------+--------------+-------------+
> > > >         ^                             ^
> > > >         |                             |
> > > >  data_hard_start                    data
> > > >                                   data_meta
> > > >
> > > > If 'bpf_xdp_adjust_meta' repositions the 'data_meta' pointer a li=
ttle
> > > > towards 'data_hard_start', it would point to the inline header, t=
hus
> > > > potentially allowing the XDP program to access the inline header.=


Fairly late to the thread sorry. Given above layout does it make sense to=

just delay extraction to the kfunc as suggested above? Sure the XDP progr=
am
could smash the entry in virtio header, but this is already the case for
anything else there. A program writing over the virtio header is likely
buggy anyways. Worse that might happen is bad rss values and mappings?

I like seeing more use cases for the hints though.

Thanks!
John=