From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vivek Goyal Subject: Re: kexec/kdump of a kvm guest? Date: Thu, 24 Jul 2008 09:15:52 -0400 Message-ID: <20080724131552.GA22243@redhat.com> References: <170fa0d20806261158me6f81d0p2ea4fa6b7e694a5a@mail.gmail.com> <486F58EC.6020408@qumranet.com> <170fa0d20807231713t19b5bd5bt5f418f509040b7aa@mail.gmail.com> <170fa0d20807240449n3e52c002qdb9c2626a192b0f1@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Alexander Graf , kexec@lists.infradead.org, kvm@vger.kernel.org, Avi Kivity To: Mike Snitzer Return-path: Received: from mx1.redhat.com ([66.187.233.31]:42905 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753812AbYGXNQF (ORCPT ); Thu, 24 Jul 2008 09:16:05 -0400 Content-Disposition: inline In-Reply-To: <170fa0d20807240449n3e52c002qdb9c2626a192b0f1@mail.gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Jul 24, 2008 at 07:49:59AM -0400, Mike Snitzer wrote: > On Thu, Jul 24, 2008 at 4:39 AM, Alexander Graf wrote: > > > > On Jul 24, 2008, at 2:13 AM, Mike Snitzer wrote: > > > >> On Sat, Jul 5, 2008 at 7:20 AM, Avi Kivity wrote: > >>> > >>> Mike Snitzer wrote: > >>>> > >>>> My host is x86_64 RHEL5U1 running 2.6.25.4 with kvm-70 (kvm-intel). > >>>> > >>>> When I configure kdump in the guest (running 2.6.22.19) and force a > >>>> crash (with 'echo c > /proc/sysrq-trigger) kexec boots the kdump > >>>> kernel but then the kernel hangs (before it gets to /sbin/init et al). > >>>> On the host, the associated qemu is consuming 100% cpu. > >>>> > >>>> I really need to be able to collect vmcores from my kvm guests. So > >>>> far I can't (on raw hardware all works fine). > >>>> > >>>> > >>> > >>> I've tested this a while ago and it worked (though I tested regular > >>> kexecs, > >>> not crashes); this may be a regression. > >>> > >>> Please run kvm_stat to see what's happening at the time of the crash. > >> > >> OK, I can look into kvm_stat but I just discovered that just having > >> kvm-intel and kvm loaded into my 2.6.22.19 kernel actually prevents > > > > Is 2.6.22.19 your host or your guest kernel? It's very unlikely that you > > loaded kvm modules in the guest. > > Correct, 2.6.22.19 is my host kernel. > > >> the host from being able to kexec/kdump too!? I didn't have any > >> guests running (only the kvm modules were loaded). As soon as I > >> unloaded the kvm modules kdump worked as expected. > >> > >> Something about kvm is completely breaking kexec/kdump on both the > >> host and guest kernels. > > > > I guess the kexec people would be pretty interested in this as well, so I'll > > just CC them for now. > > As you're stating that the host kernel breaks with kvm modules loaded, maybe > > someone there could give a hint. > > OK, I can try using a newer kernel on the host too (e.g. 2.6.25.x) to > see how kexec/kdump of the host fairs when kvm modules are loaded. > > On the guest side of things, as I mentioned in my original post, > kexec/kdump wouldn't work within a 2.6.22.19 guest with the host > running 2.6.25.4 (with kvm-70). > Hi Mike, I have never tried kexec/kdump inside a kvm guest. So I don't know if historically they have been working or not. Having said that, Why do we need kdump to work inside the guest? In this case qemu should be knowing about the memory of guest kernel and should be able to capture a kernel crash dump? I am not sure if qemu already does that. If not, then probably we should think about it? To me, kdump is a good solution for baremetal but not for virtualized environment where we already have another piece of software running which can do the job for us. We will end up wasting memory in every instance of guest (memory reserved for kdump kernel in every guest). It will be interesting to look at your results with 2.6.25.x kernels with kvm module inserted. Currently I can't think what can possibly be wrong. Thanks Vivek