From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: [PATCHv2 net-next] xen-netback: remove unconditional __pskb_pull_tail() in guest Tx path Date: Wed, 5 Nov 2014 10:57:42 +0000 Message-ID: <1415185062.11486.85.camel@citrix.com> References: <1415184622-19421-1-git-send-email-david.vrabel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: , , Wei Liu , Malcolm Crossley To: David Vrabel Return-path: Received: from smtp.citrix.com ([66.165.176.89]:58567 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754126AbaKEK5p (ORCPT ); Wed, 5 Nov 2014 05:57:45 -0500 In-Reply-To: <1415184622-19421-1-git-send-email-david.vrabel@citrix.com> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 2014-11-05 at 10:50 +0000, David Vrabel wrote: > From: Malcolm Crossley > > 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. Thanks for adding these. > 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 > Signed-off-by: David Vrabel Acked-by: Ian Campbell