[libvirt] Re: kexec/kdump of a kvm guest?
On Thu, Jul 24, 2008 at 9:15 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
On Thu, Jul 24, 2008 at 07:49:59AM -0400, Mike Snitzer wrote:
On Thu, Jul 24, 2008 at 4:39 AM, Alexander Graf <agraf@suse.de> wrote:
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.
Avi indicated he seems to remember that at least kexec worked last he tried (didn't provide when/what he tried though).
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).
I haven't looked into what mechanics qemu provides for collecting the entire guest memory image; I'll dig deeper at some point. It seems the libvirt mid-layer ("virsh dump" - dump the core of a domain to a file for analysis) doesn't support saving a kvm guest core: # virsh dump guest10 guest10.dump libvir: error : this function is not supported by the hypervisor: virDomainCoreDump error: Failed to core dump domain guest10 to guest10.dump Seems that libvirt functionality isn't available yet with kvm (I'm using libvirt 0.4.2, I'll give libvirt 0.4.4 a try). cc'ing the libvirt-list to get their insight. That aside, having the crash dump collection be multi-phased really isn't workable (that is if it requires a crashed guest to be manually saved after the fact). The host system _could_ be rebooted; whereby losing the guest's core image. So automating qemu and/or libvirtd to trigger a dump would seem worthwhile (maybe its already done?). So while I agree with you its ideal to not have to waste memory in each guest for the purposes of kdump; if users want to model a guest image as closely as possible to what will be deployed on bare metal it really would be ideal to support a 1:1 functional equivalent with kvm. I work with people who refuse to use kvm because of the lack of kexec/kdump support. I can do further research but welcome others' insight: do others have advice on how best to collect a crashed kvm guest's core?
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.
If the host's 2.6.25.4 kernel has both the kvm and kvm-intel modules loaded kexec/kdump does _not_ work (simply hangs the system). If I only have the kvm module loaded kexec/kdump works as expected (likewise if no kvm modules are loaded at all). So it would appear that kvm-intel and kexec are definitely mutually exclusive at the moment (at least on both 2.6.22.x and 2.6.25.x). Mike
Mike Snitzer wrote:
On Thu, Jul 24, 2008 at 9:15 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
On Thu, Jul 24, 2008 at 07:49:59AM -0400, Mike Snitzer wrote:
On Thu, Jul 24, 2008 at 4:39 AM, Alexander Graf <agraf@suse.de> wrote:
I can do further research but welcome others' insight: do others have advice on how best to collect a crashed kvm guest's core?
I don't know what you do in libvirt, but you can start a gdbstub in QEMU, connect with gdb, and then have gdb dump out a core. Regards, Anthony Liguori
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.
If the host's 2.6.25.4 kernel has both the kvm and kvm-intel modules loaded kexec/kdump does _not_ work (simply hangs the system). If I only have the kvm module loaded kexec/kdump works as expected (likewise if no kvm modules are loaded at all). So it would appear that kvm-intel and kexec are definitely mutually exclusive at the moment (at least on both 2.6.22.x and 2.6.25.x).
Mike -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Jul 24, 2008 at 03:03:33PM -0400, Mike Snitzer wrote:
On Thu, Jul 24, 2008 at 9:15 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
On Thu, Jul 24, 2008 at 07:49:59AM -0400, Mike Snitzer wrote:
On Thu, Jul 24, 2008 at 4:39 AM, Alexander Graf <agraf@suse.de> wrote:
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.
Avi indicated he seems to remember that at least kexec worked last he tried (didn't provide when/what he tried though).
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).
I haven't looked into what mechanics qemu provides for collecting the entire guest memory image; I'll dig deeper at some point. It seems the libvirt mid-layer ("virsh dump" - dump the core of a domain to a file for analysis) doesn't support saving a kvm guest core: # virsh dump guest10 guest10.dump libvir: error : this function is not supported by the hypervisor: virDomainCoreDump error: Failed to core dump domain guest10 to guest10.dump
Seems that libvirt functionality isn't available yet with kvm (I'm using libvirt 0.4.2, I'll give libvirt 0.4.4 a try). cc'ing the libvirt-list to get their insight.
That aside, having the crash dump collection be multi-phased really isn't workable (that is if it requires a crashed guest to be manually saved after the fact). The host system _could_ be rebooted; whereby losing the guest's core image. So automating qemu and/or libvirtd to trigger a dump would seem worthwhile (maybe its already done?).
That's a good point. Ideally, one would like dump to be captured automatically if kernel crashes and then reboot back to production kernel. I am not sure what can we do to let qemu know after crash so that it can automatically save dump. What happens in the case of xen guests. Is dump automatically captured or one has to force the dump capture externally.
So while I agree with you its ideal to not have to waste memory in each guest for the purposes of kdump; if users want to model a guest image as closely as possible to what will be deployed on bare metal it really would be ideal to support a 1:1 functional equivalent with kvm.
Agreed. Making kdump work inside kvm guest does not harm.
I work with people who refuse to use kvm because of the lack of kexec/kdump support.
Interesting.
I can do further research but welcome others' insight: do others have advice on how best to collect a crashed kvm guest's core?
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.
If the host's 2.6.25.4 kernel has both the kvm and kvm-intel modules loaded kexec/kdump does _not_ work (simply hangs the system). If I only have the kvm module loaded kexec/kdump works as expected (likewise if no kvm modules are loaded at all). So it would appear that kvm-intel and kexec are definitely mutually exclusive at the moment (at least on both 2.6.22.x and 2.6.25.x).
Ok. So first task is to fix host kexec/kdump with kvm-intel module inserted. Can you do little debugging to find out where system hangs. I generally try few things for kexec related issue debugging. 1. Specify earlyprintk= parameter for second kernel and see if control is reaching to second kernel. 2. Otherwise specify --console-serial parameter on "kexec -l" commandline and it should display a message "I am in purgatory" on serial console. This will just mean that control has reached at least till purgatory. 3. If that also does not work, then most likely first kernel itself got stuck somewhere and we need to put some printks in first kernel to find out what's wrong. Thanks Vivek
Vivek Goyal wrote:
Seems that libvirt functionality isn't available yet with kvm (I'm using libvirt 0.4.2, I'll give libvirt 0.4.4 a try). cc'ing the libvirt-list to get their insight.
That aside, having the crash dump collection be multi-phased really isn't workable (that is if it requires a crashed guest to be manually saved after the fact). The host system _could_ be rebooted; whereby losing the guest's core image. So automating qemu and/or libvirtd to trigger a dump would seem worthwhile (maybe its already done?).
That's a good point. Ideally, one would like dump to be captured automatically if kernel crashes and then reboot back to production kernel. I am not sure what can we do to let qemu know after crash so that it can automatically save dump.
We can expose a virtual pci device that when accessed, causes qemu to dump the guest's core.
Ok. So first task is to fix host kexec/kdump with kvm-intel module inserted.
Can you do little debugging to find out where system hangs. I generally try few things for kexec related issue debugging.
1. Specify earlyprintk= parameter for second kernel and see if control is reaching to second kernel.
2. Otherwise specify --console-serial parameter on "kexec -l" commandline and it should display a message "I am in purgatory" on serial console. This will just mean that control has reached at least till purgatory.
3. If that also does not work, then most likely first kernel itself got stuck somewhere and we need to put some printks in first kernel to find out what's wrong.
kvm has a reboot notifier to turn off vmx when rebooting. See kvm_reboot_notifier and kvm_reboot(). Maybe something similar is needed for kexec? -- error compiling committee.c: too many arguments to function
participants (4)
-
Anthony Liguori -
Avi Kivity -
Mike Snitzer -
Vivek Goyal