netdev - Re: [PATCH net-next 0/6] kcm: Kernel Connection Multiplexor (KCM)

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Message-Id: <20151124.104915.1283576168723043344.davem@davemloft.net>
Date:	Tue, 24 Nov 2015 10:49:15 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	tom@...bertland.com, netdev@...r.kernel.org, kernel-team@...com,
	davewatson@...com, alexei.starovoitov@...il.com
Subject: Re: [PATCH net-next 0/6] kcm: Kernel Connection Multiplexor (KCM)

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Tue, 24 Nov 2015 12:25:39 +0100

> Hello,
> 
> David Miller <davem@...emloft.net> writes:
> 
>> From: Tom Herbert <tom@...bertland.com>
>> Date: Mon, 23 Nov 2015 09:33:44 -0800
>>
>>> The TCP PSH flag is not defined for message delineation (neither is
>>> urgent pointer). We can't change that (many people have tried to add
>>> message semantics to TCP protocol but have always failed miserably).
>>
>> Agreed.
>>
>> My only gripe with kcm right now is a lack of a native sendpage.
>> We should be able to zero copy data through KCM streams without
>> any problems whatsoever.
> 
> I understood from Tom's last mail that the messages are being
> constructed *in kernel memory* before sending out of the tcp
> socket. What advantage gives sendpage? The message construction must
> actually happen before to fill in the necessary headers for the length,
> so receiver can dissect again. Streaming semantics don't really fit
> here?

You can sendpage part of a filesystem file, and push the headers in
front (the "KCM stuff") without disturbing the page frags.
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