netdev - Re: [PATCHv2 net-next] xen-netback: remove unconditional __pskb_pull

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Date:	Thu, 06 Nov 2014 14:40:57 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	david.vrabel@...rix.com
Cc:	netdev@...r.kernel.org, xen-devel@...ts.xenproject.org,
	ian.campbell@...rix.com, wei.liu2@...rix.com,
	malcolm.crossley@...rix.com
Subject: Re: [PATCHv2 net-next] xen-netback: remove unconditional
 __pskb_pull_tail() in guest Tx path

From: David Vrabel <david.vrabel@...rix.com>
Date: Wed, 5 Nov 2014 10:50:22 +0000

> From: Malcolm Crossley <malcolm.crossley@...rix.com>
> 
> Unconditionally pulling 128 bytes into the linear area is not required
> for:
> 
> - security: Every protocol demux starts with pskb_may_pull() to pull
>   frag data into the linear area, if necessary, before looking at
>   headers.
> 
> - performance: Netback has already grant copied up-to 128 bytes from
>   the first slot of a packet into the linear area. The first slot
>   normally contain all the IPv4/IPv6 and TCP/UDP headers.
> 
> The unconditional pull would often copy frag data unnecessarily.  This
> is a performance problem when running on a version of Xen where grant
> unmap avoids TLB flushes for pages which are not accessed.  TLB
> flushes can now be avoided for > 99% of unmaps (it was 0% before).
> 
> Grant unmap TLB flush avoidance will be available in a future version
> of Xen (probably 4.6).
> 
> Signed-off-by: Malcolm Crossley <malcolm.crossley@...rix.com>
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>

Applied, thanks.
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