netdev - Re: [PATCH net RFC] xen-netback: Fix grant ref resolution in RX path

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Message-ID: <53749381.7010301@citrix.com>
Date:	Thu, 15 May 2014 11:14:25 +0100
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	<xen-devel@...ts.xenproject.org>, <wei.liu2@...rix.com>,
	<linux@...elenboom.it>, <paul.durrant@...rix.com>,
	<netdev@...r.kernel.org>, <david.vrabel@...rix.com>,
	<davem@...emloft.net>
Subject: Re: [PATCH net RFC] xen-netback: Fix grant ref resolution in RX path

On 15/05/14 09:31, Ian Campbell wrote:
> On Wed, 2014-05-14 at 12:12 +0100, Zoltan Kiss wrote:
>>> The 'cleanup' of stale ubufs should be right after __pskb_pull_tail().
>> We can't fix every place in the kernel where frags might be changed,
>> especially with a netback specific stuff, so unfortunately that won't work
>
> Is it worth fixing up the ones in netback though so that the things
> injected into the stack are consistent when we hand them over? It would
> avoid some search overhead on the rx path at the other end I guess?
> Perhaps not significant though.

There are plans to remove that unconditional pull, as it damages 
performance. It is better to use during checksum setup maybe_pull_tail, 
and pull up whatever is needed for checksum setup (this is already done, 
partially). A sensible netfront would send the header in the first slot 
anyway, so netback won't pull, and it definitely won't pull the whole 
first frag.

Regards,

Zoli
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