netdev - Re: [PATCH net-next 8/8] virtio_net: introduce receive_small

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Date:   Fri, 31 Mar 2023 03:24:56 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     Paolo Abeni <pabeni@...hat.com>, Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next 8/8] virtio_net: introduce receive_small_xdp()

On Fri, Mar 31, 2023 at 03:20:35PM +0800, Xuan Zhuo wrote:
> On Thu, 30 Mar 2023 12:48:22 +0200, Paolo Abeni <pabeni@...hat.com> wrote:
> > On Tue, 2023-03-28 at 20:04 +0800, Xuan Zhuo wrote:
> > > @@ -949,15 +1042,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
> > >  {
> > >  	struct sk_buff *skb;
> > >  	struct bpf_prog *xdp_prog;
> > > -	unsigned int xdp_headroom = (unsigned long)ctx;
> > > -	unsigned int header_offset = VIRTNET_RX_PAD + xdp_headroom;
> > > +	unsigned int header_offset = VIRTNET_RX_PAD;
> > >  	unsigned int headroom = vi->hdr_len + header_offset;
> >
> > This changes (reduces) the headroom for non-xpd-pass skbs.
> >
> > [...]
> > > +	buf += header_offset;
> > > +	memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
> >
> > AFAICS, that also means that receive_small(), for such packets, will
> > look for the virtio header in a different location. Is that expected?
> 
> 
> That is a mistake.
> 
> Will fix.
> 
> Thanks.

Do try to test small and big packet configurations though, too.

> >
> > Thanks.
> >
> > Paolo
> >