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

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Message-ID: <5431EDF7.3080201@gmail.com>
Date:	Sun, 05 Oct 2014 18:18:47 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	David Miller <davem@...emloft.net>
CC:	fw@...len.de, 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

On 10/05/2014 06:09 PM, David Miller wrote:
> 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.
>

Right, my description could have been better and called this out.

Thanks.

-- 
John Fastabend         Intel Corporation
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