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

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Message-Id: <20141005.210938.412085917241295226.davem@davemloft.net>
Date:	Sun, 05 Oct 2014 21:09:38 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	fw@...len.de
Cc:	john.fastabend@...il.com, dborkman@...hat.com,
	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

From: Florian Westphal <fw@...len.de>
Date: Mon, 6 Oct 2014 02:29:51 +0200

> 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'?

FWIW, it also avoids the domain switch (which is just a fancy way
to refer to performing the system call), both in and out.
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