Subject: Re: [PATCH net-next 2/2] virtio-net: simplify XDP handling in small buffer

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 1 Mar 2018 09:02:06 +0100

On Thu,  1 Mar 2018 11:19:05 +0800 Jason Wang <[email-masked]> wrote:

> We used to do data copy through xdp_linearize_page() for the buffer
> without sufficient headroom, it brings extra complexity without
> helping for the performance. So this patch remove it and switch to use
> generic XDP routine to handle this case.

I don't like where this is going.  I don't like intermixing the native
XDP and generic XDP in this way, for several reasons:

1. XDP generic is not feature complete, e.g. cpumap will drop these
   packets. It might not be possible to implement some features, think
   of (AF_XDP) zero-copy.

2. This can easily cause out-of-order packets.

3. It makes it harder to troubleshoot, when diagnosing issues
   around #1, we have a hard time determining what path an XDP packet
   took (the xdp tracepoints doesn't know).

[...]
> @@ -590,25 +526,14 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  		if (unlikely(hdr->hdr.gso_type))
>  			goto err_xdp;
>  
> +		/* This happnes when headroom is not enough because
> +		 * the buffer was refilled before XDP is set. This
> +		 * only happen for several packets, for simplicity,
> +		 * offload them to generic XDP routine.

In my practical tests, I also saw that sometime my ping packets were
traveling this code-path, even after a long time when XDP were attached.

This worries me a bit, for troubleshooting purposes... as this can give
a strange user experience given point #1.

> +		 */
>  		if (unlikely(xdp_headroom < virtnet_get_headroom(vi))) {
> -			int offset = buf - page_address(page) + header_offset;
> -			unsigned int tlen = len + vi->hdr_len;
> -			u16 num_buf = 1;

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