Linux Virtualization - Mar 2023 - [PATCH net-next 8/8] virtio_net: introduce receive_small_xdp()

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?

Thanks.

Paolo

On Thu, 30 Mar 2023 12:48:22 +0200, Paolo Abeni <pabeni at redhat.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.
>
> Thanks.
>
> Paolo
>