netdev - Re: [RFC PATCH bpf-next 05/12] xdp: add MEM_TYPE_ZERO

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Date:   Thu, 17 May 2018 07:57:56 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     magnus.karlsson@...il.com, magnus.karlsson@...el.com,
        alexander.h.duyck@...el.com, alexander.duyck@...il.com,
        john.fastabend@...il.com, ast@...com,
        willemdebruijn.kernel@...il.com, daniel@...earbox.net,
        mst@...hat.com, netdev@...r.kernel.org,
        Björn Töpel <bjorn.topel@...el.com>,
        michael.lundkvist@...csson.com, jesse.brandeburg@...el.com,
        anjali.singhai@...el.com, qi.z.zhang@...el.com,
        intel-wired-lan@...ts.osuosl.org, brouer@...hat.com
Subject: Re: [RFC PATCH bpf-next 05/12] xdp: add MEM_TYPE_ZERO_COPY

On Tue, 15 May 2018 21:06:08 +0200
Björn Töpel <bjorn.topel@...il.com> wrote:

> @@ -82,6 +88,10 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
>  	int metasize;
>  	int headroom;
>  
> +	// XXX implement clone, copy, use "native" MEM_TYPE
> +	if (xdp->rxq->mem.type == MEM_TYPE_ZERO_COPY)
> +		return NULL;
> +

There is going to be significant tradeoffs between AF_XDP zero-copy and
copy-variant.  The copy-variant, still have very attractive
RX-performance, and other benefits like no exposing unrelated packets
to userspace (but limit these to the XDP filter).

Thus, as a user I would like to choose between AF_XDP zero-copy and
copy-variant. Even if my NIC support zero-copy, I can be interested in
only enabling the copy-variant. This patchset doesn't let me choose.

How do we expose this to userspace?
(Maybe as simple as an sockaddr_xdp->sxdp_flags flag?)

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