Re: [Qemu-devel] [PATCH 0/2 v3] kvm: notify host when guest panicked

[Wen Congyang]

At 03/22/2012 03:28 PM, Gleb Natapov Wrote:
> On Wed, Mar 21, 2012 at 02:19:34PM -0500, Anthony Liguori wrote:
>> On 03/21/2012 11:25 AM, Avi Kivity wrote:
>>> On 03/21/2012 06:18 PM, Corey Minyard wrote:
>>>>
>>>>> Look at drivers/char/ipmi/ipmi_msghandler.c. It has code to send panic
>>>>> event over IMPI. The code is pretty complex. Of course if we a going to
>>>>> implement something more complex than simple hypercall for panic
>>>>> notification we better do something more interesting with it than just
>>>>> saying "panic happened", like sending stack traces on all cpus for
>>>>> instance.
>>>>
>>>> I doubt that's the best example, unfortunately.  The IPMI event log
>>>> has limited space and it has to be send a little piece at a time since
>>>> each log entry is 14 bytes.  It just prints the panic string, nothing
>>>> else.  Not that it isn't useful, it has saved my butt before.
>>>>
>>>> You have lots of interesting options with paravirtualization.  You
>>>> could, for instance, create a console driver that delivered all
>>>> console output efficiently through a hypercall.  That would be really
>>>> easy.  Or, as you mention, a custom way to deliver panic information.
>>>> Collecting information like stack traces would be harder to
>>>> accomplish, as I don't think there is currently a way to get it except
>>>> by sending it to printk.
>>>
>>> That already exists; virtio-console (or serial console emulation) can do
>>> the job.
>>
>> I think the use case here is pretty straight forward: if the guest
>> finds itself in bad place, it wants to indicate that to the host.
>>
>> We shouldn't rely on any device drivers or complex code.  It should
>> be as close to a single instruction as possible that can run even if
>> interrupts are disabled.
>>
>> An out instruction fits this very well.  I think a simple protocol like:
>>
>> inl PORT -> returns a magic number indicating the presence of qemucalls
>> inl PORT+1 -> returns a bitmap of supported features
>>
> Sigh, one more PV isa device.
> 
>> outl PORT+1 -> data reg1
>> outl PORT+2 -> data reg2
>> outl PORT+N -> data regN
>>
>> outl PORT -> qemucall of index value with arguments 1..N
> And you think you can trust panicked SMP guest to not call this on
> multiple cpus simultaneously?

We can register panic notifier in the guest kernel, and do it in the panic
notifier callback.

Thanks
Wen Congyang

> 
>>
>> Regards,
>>
>> Anthony Liguori
>>
>>>
>>> In fact the feature can be implemented 100% host side by searching for a
>>> panic string signature in the console logs.
>>>
> 
> --
>                       Gleb.
> 
>