From mboxrd@z Thu Jan 1 00:00:00 1970 From: Glauber Costa Subject: Re: [PATCH 3/9] change order of kvm_init call. Date: Mon, 27 Jul 2009 15:20:08 -0300 Message-ID: <20090727182008.GH4776@poweredge.glommer> References: <1248131416-11272-1-git-send-email-glommer@redhat.com> <1248131416-11272-2-git-send-email-glommer@redhat.com> <1248131416-11272-3-git-send-email-glommer@redhat.com> <1248131416-11272-4-git-send-email-glommer@redhat.com> <4A6CA7A0.8020901@web.de> <20090727173851.GC4776@poweredge.glommer> <4A6DE892.7050801@us.ibm.com> <20090727180003.GE4776@poweredge.glommer> <4A6DED90.3010200@web.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Anthony Liguori , kvm@vger.kernel.org, avi@redhat.com, markmc@redhat.com, berrange@redhat.com To: Jan Kiszka Return-path: Received: from mx2.redhat.com ([66.187.237.31]:52006 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751919AbZG0SNL (ORCPT ); Mon, 27 Jul 2009 14:13:11 -0400 Content-Disposition: inline In-Reply-To: <4A6DED90.3010200@web.de> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Jul 27, 2009 at 08:10:24PM +0200, Jan Kiszka wrote: > Glauber Costa wrote: > > On Mon, Jul 27, 2009 at 12:49:06PM -0500, Anthony Liguori wrote: > >> Glauber Costa wrote: > >>> On Sun, Jul 26, 2009 at 08:59:44PM +0200, Jan Kiszka wrote: > >>> > >>>> Glauber Costa wrote: > >>>> > >>>>> The goal is to get rid of the call to kvm_init. But those things > >>>>> are subtle, and often break. So do it in a separate patch, to help > >>>>> finding potential issues in future bisections. > >>>>> > >>>> Found such an issued: This patch triggers a segfault if no kvm modules > >>>> are loaded and you start qemu without -no-kvm. Please have a look. > >>>> > >>>> Jan > >>>> > >>> ok, the culprit seems to be a > >>> > >>> if (kvm_enabled()) > >>> return; > >>> > >>> in the beginning of code_gen_alloc. > >>> > >>> It is 7f3d0cbe, by Avi, and according to changelog, suggested by anthony. > >>> I however, fail to realise the purpose of this optimization. For one thing, > >>> it totally dictates that kvm has absolutely to be enabled or disabled prior > >>> to this point. No mind changing later. Also, the real deal is to be able > >>> to compile out tcg entirely. The strategy of just disabling the code gen > >>> alloc is a minor nitpick that just papers over this. > >>> > >> I agree with you in principle but I think reverting this papers over an > >> issue. > >> > >> Why are we touching code_gen_ptr when using KVM? Can someone post the > >> full back trace? > > we're not. > > > > The issue happens exactly when the kvm modules are not loaded, then we're failing > > to initialize kvm. However, in the patch that raised this issue, I'm moving > > KVM initialization to after this code path. And in qemu-kvm.git, kvm is > > enabled-by-default. So tcg code would think kvm is enabled and skip initialization, > > while kvm code will fail to really initialize itself later. > > > > Result? Mayhem. > > > > I think we should simply resolves this the way upstream does: Do not > start if modules are missing and -no-kvm is omitted - or even switch > over to -enable-kvm as I think you already suggested in some other > thread. Then we can either fail or succeed, but not fall back more or > less silently. This falling back of qemu-kvm to tcg is a constant source > of confusion anyway. switching to --enable-kvm would be my preferred solution, but guys from mgmt tools may not like it.