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

[Wen Congyang]

At 03/22/2012 03:19 AM, 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:

This solution is more simple than using virtio-serial.

> 
> inl PORT -> returns a magic number indicating the presence of qemucalls

I donot understantd this instruction's purpose.

> inl PORT+1 -> returns a bitmap of supported features

Hmm, we can execute this instruction when guest starts. If the userspace
does not process panicked event, there is no need to notify it.

> 
> outl PORT+1 -> data reg1
> outl PORT+2 -> data reg2
> outl PORT+N -> data regN

We can get the register value from vmcs. So there is no need to tell
the register value to the host.

If we decide to avoid touching hypervisor, I agree with this solution.

Thanks
Wen Congyang
> 
> outl PORT -> qemucall of index value with arguments 1..N
> 
> Regards,
> 
> Anthony Liguori
> 
>>
>> In fact the feature can be implemented 100% host side by searching for a
>> panic string signature in the console logs.
>>
> 
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