From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932982Ab2JCQPq (ORCPT ); Wed, 3 Oct 2012 12:15:46 -0400 Received: from smtp.ctxuk.citrix.com ([62.200.22.115]:22973 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932619Ab2JCQPo (ORCPT ); Wed, 3 Oct 2012 12:15:44 -0400 X-IronPort-AV: E=Sophos;i="4.80,528,1344211200"; d="scan'208";a="14920864" Message-ID: <1349280814.650.178.camel@zakaz.uk.xensource.com> Subject: Re: [Xen-devel] [PATCH] xen: point xen_start_info to a dummy struct for PV on HVM guests From: Ian Campbell To: Stefano Stabellini CC: Konrad Rzeszutek Wilk , "xen-devel@lists.xensource.com" , "linux-kernel@vger.kernel.org" Date: Wed, 3 Oct 2012 17:13:34 +0100 In-Reply-To: References: <1349272182.650.150.camel@zakaz.uk.xensource.com> <1349272482.650.151.camel@zakaz.uk.xensource.com> <20121003141116.GA10633@phenom.dumpdata.com> <1349276431.650.156.camel@zakaz.uk.xensource.com> <1349279862.650.171.camel@zakaz.uk.xensource.com> Organization: Citrix Systems, Inc. Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.3-1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2012-10-03 at 17:05 +0100, Stefano Stabellini wrote: > On Wed, 3 Oct 2012, Ian Campbell wrote: > > On Wed, 2012-10-03 at 16:48 +0100, Stefano Stabellini wrote: > > > On Wed, 3 Oct 2012, Ian Campbell wrote: > > > > On Wed, 2012-10-03 at 15:11 +0100, Konrad Rzeszutek Wilk wrote: > > > > > On Wed, Oct 03, 2012 at 02:54:42PM +0100, Ian Campbell wrote: > > > > > > On Wed, 2012-10-03 at 14:51 +0100, Stefano Stabellini wrote: > > > > > > > On Wed, 3 Oct 2012, Ian Campbell wrote: > > > > > > > > On Wed, 2012-10-03 at 14:37 +0100, Stefano Stabellini wrote: > > > > > > > > > PV on HVM guests don't have a start_info page mapped by Xen, so > > > > > > > > > xen_start_info is just NULL for them. > > > > > > > > > That is problem because other parts of the code expect xen_start_info to > > > > > > > > > point to something valid, for example xen_initial_domain() is defined as > > > > > > > > > follow: > > > > > > > > > > > > > > > > > > #define xen_initial_domain() (xen_domain() && \ > > > > > > > > > xen_start_info->flags & SIF_INITDOMAIN) > > > > > > > > > > > > > > > > But anyone who calls this before xen_start_info is setup is going to get > > > > > > > > a bogus result, specifically in this case they will think they are domU > > > > > > > > when in reality they are dom0 -- wouldn't it be better to fix those > > > > > > > > callsites? > > > > > > > > > > > > > > That cannot be the case because setting up xen_start_info is the very > > > > > > > first thing that is done, before even calling to C. > > > > > > > > > > > > On PV, yes, but you are trying to fix PVHVM here, no? > > > > > > > > > > > > Otherwise if this is always set before calling into C then what is the > > > > > > purpose of this patch? > > > > > > > > > > to fix this - as PVHVM has it set to NULL and we end up de-referencing > > > > > the xen_start_info and crashing. As so:: > > > > > > > > > > > > > Right, so returning to my original point: The caller here is calling > > > > xen_initial_domain() *before* start info is setup. This is bogus and is > > > > your actual bug, all this patch does is hide that real issue. > > > > > > That is because xen_start_info wasn't setup at all for PV on HVM guests. > > > > > > The real reason is that PV on HVM guests don't have one, but that is > > > another matter. Until we get rid of all the references to xen_start_info > > > outside of PV specific code, we should just assume that there is one, > > > and that is already setup. > > > > > > One day not too far from now, we might refactor the code to never > > > reference xen_start_info directly, but I don't think that now is the > > > time for that. Also consider that this is the same thing we do on ARM. > > > > We actual fill in the dummy start info with valid information on ARM > > though, we don't just leave it full of zeroes. > > > > If we do start out with start_info pointing to an uninitialised > > start_info on ARM too then I would argue that this is also a mistake. > > Yes, we do point xen_start_info to an uninitialised start_info on ARM > too (I don't think is a mistake). Then when and if we have more > information we write them to start_info. So callers of xen_initial_domain in dom0 before xen_guest_init is called get the wrong result? That sounds like a mistake to me. > > We > > should leave the NULL pointer in place until we setup the content of the > > dummy start info -- exactly because the resulting crash indicates to us > > that someone has accessed the si before we've initialised it. > > I don't think so. It is initialized to zero, that is the right thing to > do. Except it isn't in the dom0 case... > > > > With this "fix" the caller of xen_initial_domain shown in this trace now > > > > gets a rubbish result based on the content of a dummy shared info > > > > instead of the real answer from that actual shared info. > > > > > > That is not true. The caller gets a zero result, that is completely > > > appropriate in this case, given that a PV on HVM guest doesn't have a > > > start_info. > > > > It's just a side effect of Linux zeroing its bss though and zero > > happening to be the right answer for a PVHVM guest in this case. > > well, I would call that "by design" ;-) Well, in that case it should be documented not just implicit! > > Is it true that zero is an appropriate result for all uses of fields in > > start_info on PVHVM? > > I think so. In fact, if we wanted to, we could have the dummy struct > initialized to something different, but I don't think that we should.