Subject: Re: [PATCH bpf-next 01/11] xdp: implement convert_to_xdp_frame for MEM_TYPE_ZERO_COPY

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 28 Aug 2018 16:11:02 +0200

On Tue, 28 Aug 2018 14:44:25 +0200
Björn Töpel <[email-masked]> wrote:

> From: Björn Töpel <[email-masked]>
> 
> This commit adds proper MEM_TYPE_ZERO_COPY support for
> convert_to_xdp_frame. Converting a MEM_TYPE_ZERO_COPY xdp_buff to an
> xdp_frame is done by transforming the MEM_TYPE_ZERO_COPY buffer into a
> MEM_TYPE_PAGE_ORDER0 frame. This is costly, and in the future it might
> make sense to implement a more sophisticated thread-safe alloc/free
> scheme for MEM_TYPE_ZERO_COPY, so that no allocation and copy is
> required in the fast-path.

This is going to be slow. Especially the dev_alloc_page() call, which
for small frames is likely going to be slower than the data copy.
I guess this is a good first step, but I do hope we will circle back and
optimize this later.  (It would also be quite easy to use
MEM_TYPE_PAGE_POOL instead to get page recycling in devmap redirect case).

I would have liked the MEM_TYPE_ZERO_COPY frame to travel one level
deeper into the redirect-core code.  Allowing devmap to send these
frame without copy, and allow cpumap to do the dev_alloc_page() call
(+copy) on the remote CPU.

> Signed-off-by: Björn Töpel <[email-masked]>
> ---
>  include/net/xdp.h |  5 +++--
>  net/core/xdp.c    | 39 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 76b95256c266..0d5c6fb4b2e2 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -91,6 +91,8 @@ static inline void xdp_scrub_frame(struct xdp_frame *frame)
>  	frame->dev_rx = NULL;
>  }
>  
> +struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
> +
>  /* Convert xdp_buff to xdp_frame */
>  static inline
>  struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
> @@ -99,9 +101,8 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
>  	int metasize;
>  	int headroom;
>  
> -	/* TODO: implement clone, copy, use "native" MEM_TYPE */
>  	if (xdp->rxq->mem.type == MEM_TYPE_ZERO_COPY)
> -		return NULL;
> +		return xdp_convert_zc_to_xdp_frame(xdp);
>  
>  	/* Assure headroom is available for storing info */
>  	headroom = xdp->data - xdp->data_hard_start;
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 89b6785cef2a..be6cb2f0e722 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -398,3 +398,42 @@ void xdp_attachment_setup(struct xdp_attachment_info *info,
>  	info->flags = bpf->flags;
>  }
>  EXPORT_SYMBOL_GPL(xdp_attachment_setup);
> +
> +struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp)
> +{
> +	unsigned int metasize, headroom, totsize;
> +	void *addr, *data_to_copy;
> +	struct xdp_frame *xdpf;
> +	struct page *page;
> +
> +	/* Clone into a MEM_TYPE_PAGE_ORDER0 xdp_frame. */
> +	metasize = xdp_data_meta_unsupported(xdp) ? 0 :
> +		   xdp->data - xdp->data_meta;
> +	headroom = xdp->data - xdp->data_hard_start;
> +	totsize = xdp->data_end - xdp->data + metasize;
> +
> +	if (sizeof(*xdpf) + totsize > PAGE_SIZE)
> +		return NULL;
> +
> +	page = dev_alloc_page();
> +	if (!page)
> +		return NULL;
> +
> +	addr = page_to_virt(page);
> +	xdpf = addr;
> +	memset(xdpf, 0, sizeof(*xdpf));
> +
> +	addr += sizeof(*xdpf);
> +	data_to_copy = metasize ? xdp->data_meta : xdp->data;
> +	memcpy(addr, data_to_copy, totsize);
> +
> +	xdpf->data = addr + metasize;
> +	xdpf->len = totsize - metasize;
> +	xdpf->headroom = 0;
> +	xdpf->metasize = metasize;
> +	xdpf->mem.type = MEM_TYPE_PAGE_ORDER0;
> +
> +	xdp_return_buff(xdp);
> +	return xdpf;
> +}
> +EXPORT_SYMBOL_GPL(xdp_convert_zc_to_xdp_frame);

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