From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: [Xen-devel] [PATCH net-next v4 2/5] xen-netback: add support for IPv6 checksum offload from guest Date: Mon, 14 Oct 2013 13:19:20 +0100 Message-ID: <525BE148.1010508@citrix.com> References: <1381503982-1418-1-git-send-email-paul.durrant@citrix.com> <1381503982-1418-3-git-send-email-paul.durrant@citrix.com> <20131014104235.GB11739@zion.uk.xensource.com> <9AAE0902D5BC7E449B7C8E4E778ABCD0138B8D@AMSPEX01CL01.citrite.net> <20131014105527.GD11739@zion.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: Paul Durrant , "netdev@vger.kernel.org" , Ian Campbell , David Vrabel , "xen-devel@lists.xen.org" To: Wei Liu Return-path: Received: from smtp.citrix.com ([66.165.176.89]:45155 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756342Ab3JNMTX (ORCPT ); Mon, 14 Oct 2013 08:19:23 -0400 In-Reply-To: <20131014105527.GD11739@zion.uk.xensource.com> Sender: netdev-owner@vger.kernel.org List-ID: On 14/10/13 11:55, Wei Liu wrote: > On Mon, Oct 14, 2013 at 11:49:20AM +0100, Paul Durrant wrote: >>> -----Original Message----- >>> From: Wei Liu [mailto:wei.liu2@citrix.com] >>> Sent: 14 October 2013 11:43 >>> To: Paul Durrant >>> Cc: xen-devel@lists.xen.org; netdev@vger.kernel.org; Wei Liu; David Vrabel; >>> Ian Campbell >>> Subject: Re: [PATCH net-next v4 2/5] xen-netback: add support for IPv6 >>> checksum offload from guest >>> >>> On Fri, Oct 11, 2013 at 04:06:19PM +0100, Paul Durrant wrote: >>> [...] >>>> -/* >>>> - * This is the amount of packet we copy rather than map, so that the >>>> - * guest can't fiddle with the contents of the headers while we do >>>> - * packet processing on them (netfilter, routing, etc). >>>> +/* This is a miniumum size for the linear area to avoid lots of >>>> + * calls to __pskb_pull_tail() as we set up checksum offsets. >>>> */ >>> >>> You seem to forget to explain why 128 is chosen. :-) >> >> Is that not sufficient explanation? What sort of thing are you looking for? >> > >>>From the second version of this patch, we had a conversation. > >> Where does 128 come from? >> > > "It's just an arbitrary power of 2 that was chosen because it seems to > cover most likely v6 headers and all v4 headers." > > So something like: "We choose 128 which is likely to cover most V6 > headers and all V4 headers" would be sufficeint. Is "most IPv6 headers" actually good enough? Don't we need to ensure netback copies all IP headers? David