Subject: Re: [PATCH net-next 20/33] vhost_net: also populate XDP frame size

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 30 Apr 2020 11:54:15 +0200

On Mon, 27 Apr 2020 13:50:15 +0800
Jason Wang <[email-masked]> wrote:

> On 2020/4/23 上午12:09, Jesper Dangaard Brouer wrote:
> > In vhost_net_build_xdp() the 'buf' that gets queued via an xdp_buff
> > have embedded a struct tun_xdp_hdr (located at xdp->data_hard_start)
> > which contains the buffer length 'buflen' (with tailroom for
> > skb_shared_info). Also storing this buflen in xdp->frame_sz, does not
> > obsolete struct tun_xdp_hdr, as it also contains a struct
> > virtio_net_hdr with other information.
> >
> > Cc: Jason Wang <[email-masked]>
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >   drivers/vhost/net.c |    1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 87469d67ede8..69af007e22f4 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -745,6 +745,7 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
> >   	xdp->data = buf + pad;
> >   	xdp->data_end = xdp->data + len;
> >   	hdr->buflen = buflen;
> > +	xdp->frame_sz = buflen;
> >   
> >   	--net->refcnt_bias;
> >   	alloc_frag->offset += buflen;  
> 
> 
> Tun_xdp_one() will use hdr->buflen as the frame_sz (patch 19), so it 
> looks to me there's no need to do this?

I was thinking to go the "other way", meaning let tun_xdp_one() use
xdp->frame_sz, which gets set here.  This would allow us to refactor
the code, and drop struct tun_xdp_hdr, as (see pahole below) it only
carries 'buflen' and the remaining part comes from struct
virtio_net_hdr, which could be used directly instead.

As this will be a code refactor, I would prefer we do it after this
patchseries is agreed upon.

$ pahole -C tun_xdp_hdr drivers/net/tap.o
struct tun_xdp_hdr {
	int                        buflen;               /*     0     4 */
	struct virtio_net_hdr gso;                       /*     4    10 */

	/* size: 16, cachelines: 1, members: 2 */
	/* padding: 2 */
	/* last cacheline: 16 bytes */
};

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer