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

[Wen Congyang]

At 03/22/2012 03:31 PM, Gleb Natapov Wrote:
> On Thu, Mar 22, 2012 at 09:05:12AM +0800, Wen Congyang wrote:
>> 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.
>>
> No device should examine register value. Ideally QEMU would read
> registers only during migration.

I mean: if the qemu(or other app) want to know the register value, it can
get it from vmcs. So there is no need to pass register value from guest
to host.

Another question: each outl will cause vmexit?

Thanks
Wen Congyang

> 
>> 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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> 
> --
> 			Gleb.
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