From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface Date: Tue, 12 Mar 2013 10:49:04 -0400 Message-ID: <20130312144904.GE17901@phenom.dumpdata.com> References: <5139A56D.30107@crc.id.au> <00bf01ce1c34$395a4f20$ac0eed60$@jacekowski.org> <20130308203636.GA5422@phenom.dumpdata.com> <000301ce1c49$a3f759d0$ebe60d70$@jacekowski.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <000301ce1c49$a3f759d0$ebe60d70$@jacekowski.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jacek Milewicz Cc: 'Ian Campbell' , 'Steven Haigh' , 'annie li' , Wei.Liu2@citrix.com, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On Fri, Mar 08, 2013 at 11:09:56PM +0100, Jacek Milewicz wrote: > > > > Is there any further information on a possible fix for this? > Thoughts? > > > > Ideas? As it seems to be hitting multiple sites, I'd like to get it > > > fixed asap... > > > > > > There are 3 ways I can see this fixed: > > > - update guests so all have same MAX_SKB_FRAGS (that includes windows > > > drivers (windows drivers use 19 for MAX_SKB_FRAGS)) -add some sort of > > > negotiation between host and guest > > > - change MAX_SKB_FRAGS to 19 to accommodate all guests > > > > > > Unfortunately first one requires changes to the guest and most don't > > > have that luxury. So the only way I see it could be fixed without > > > breaking compatibility even more is > > > > Ugh. The negotiations between host and guest is probably the best > choice. > > The issues you are going to hit are that you might need to redo the skbs > to > > match what the frontend's max is. > > > > Annie, Wei, Ian - were there some RFC patches floating around for this? > > > > As much as I agree that negotiation is the best option in long term. It > relies on people upgrading their kernels - and this is not the case (only > people running kernels from before 2011-12-23 are affected by this bug > (I'm not sure about other platforms)). > And in a lot of cases (VPS providers) there is no easy way to upgrade > guest kernels. Sure. The other option in the xen-netback might be a configurable option to let net-back know that it is running with older guests which expect 18 size MAX_SKB_FRAGS. That way we have the negotiation part, the code to deal with backend MAX_SKB_FRAGS != frontend MAX_SKB_FRAGS and the value that the system admin provides. What I am saying is that if we get the negotiation protcol figured out properly we can provide a per-guest value that the system admin can over-write to say: "I know that this guest uses _this_ size".