[libvirt] question about guest status

Wen Congyang wency at cn.fujitsu.com
Thu Feb 16 06:42:37 UTC 2012

At 02/16/2012 11:04 AM, KAMEZAWA Hiroyuki Wrote:
> On Wed, 15 Feb 2012 11:00:25 +0000
> "Daniel P. Berrange" <berrange at redhat.com> wrote:
>> On Wed, Feb 15, 2012 at 02:44:18PM +0800, Wen Congyang wrote:
>>> Hi all:
>>> If the guest runs on xen, we can know the guest OS paniced, and the guest status
>>> is paniced. But if the guest runs on qemu/kvm, we need the same feature. And the
>>> feature to know guest-is-panic is a very very good feature for enterprise.
>>> We have some ways to know that guest has paniced:
>>> 1. watchdog timeouts
>>>    It is not a good way, because we can not say "the guest is paniced" in 100% accuracy
>>> 2. use a PV driver(like the XEN)
>>>    The old RHELs do not have such driver...
>>>    It can not work for the second kernel...
>>> 3. Adding a function for console-logging (in ring buffer or some) and check messages
>>>    given by guests. If we allow some filtering script or some, we can catch Linux's
>>>    panic messages. We can say the guest paniced if we catch such messages. We may be
>>>    able to catch other events by other script
>>> I prefer to 3 now. Does anyone have a better idea?
>> I have discussed this with Anthony before and the thought was that QEMU/KVM
>> should provide some kind of paravirtualized crash notification service to
>> the guest. ie option 2.  such a device would likely be quite simple and
>> thus possible to backport to older kernels as needed
> AFAIK, linux kernel has notifier at panic but it doesn't work if kdump is enabled.
> And IIRC, There was a discussion in lkml that any modules should not be called when

If CONFIG_KVM_GUEST is yes, the kernel can know that it runs on kvm. The detecting code
is also built into kernel. So if we decide to use PV driver, we can put the code
in arch/x86/kerne/kvm.c.

> crash_kexec() is called.  So, no chance to notify it from the module in the 1st kernel, now.
> Calling it from 2nd kernel will work but the bahavior will be different from
> non-kdump guest.
> Can't we do notify by hypercall ? But this will require guest modification..

Yes, xen requires the same modification.

Wen Congyang

> Thanks,
> -Kame

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