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

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Message-ID: <5431EC82.7010305@gmail.com>
Date:	Sun, 05 Oct 2014 18:12:34 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	Florian Westphal <fw@...len.de>
CC:	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 05:29 PM, Florian Westphal wrote:
> 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.
>

Well it was only for convenience if it doesn't fit as a socket
option we can remove it. We can look up the device using the netdev
name from the bind call. I see your point though so if there is
consensus that this is not needed that is fine.

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

This was likely a poor description. If you want to let user space
poll on the ring (without using system calls or interrupts) then
I don't see how you can _not_ expose the ring directly complete with
the vendor descriptor formats.

> Thanks,
> Florian
>


-- 
John Fastabend         Intel Corporation
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