netdev - Re: [PATCHv1 net-next] xen-netback: remove unconditional pull_skb

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Date:	Tue, 04 Nov 2014 16:41:13 -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: [PATCHv1 net-next] xen-netback: remove unconditional
 pull_skb_tail in guest Tx path

From: David Vrabel <david.vrabel@...rix.com>
Date: Mon, 3 Nov 2014 17:23:51 +0000

> From: Malcolm Crossley <malcolm.crossley@...rix.com>
> 
> Unconditionally pulling 128 bytes into the linear buffer is not
> required. Netback has already grant copied up-to 128 bytes from the
> first slot of a packet into the linear buffer. 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>

Now that this has been discussed a bit, it is possible to get an ack or two?

Thanks.
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