netdev - Re: [net-next PATCH v1 1/3] net: sched: af_packet support for direct ring access

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Message-ID: <20141006002951.GA24376@breakpoint.cc>
Date:	Mon, 6 Oct 2014 02:29:51 +0200
From:	Florian Westphal <fw@...len.de>
To:	John Fastabend <john.fastabend@...il.com>
Cc:	dborkman@...hat.com, fw@...len.de, gerlitz.or@...il.com,
	hannes@...essinduktion.org, netdev@...r.kernel.org,
	john.ronciak@...el.com, amirv@...lanox.com, eric.dumazet@...il.com,
	danny.zhou@...el.com
Subject: Re: [net-next PATCH v1 1/3] net: sched: af_packet support for direct
 ring access

John Fastabend <john.fastabend@...il.com> wrote:
> There is one critical difference when running with these interfaces
> vs running without them. In the normal case the af_packet module
> uses a standard descriptor format exported by the af_packet user
> space headers. In this model because we are working directly with
> driver queues the descriptor format maps to the descriptor format
> used by the device. User space applications can learn device
> information from the socket option PACKET_DEV_DESC_INFO which
> should provide enough details to extrapulate the descriptor formats.
> Although this adds some complexity to user space it removes the
> requirement to copy descriptor fields around.

I find it very disappointing that we seem to have to expose such
hardware specific details to userspace via hw-independent interface.

How big of a cost are we talking about when you say that it 'removes
the requirement to copy descriptor fields'?

Thanks,
Florian
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