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

[Wen Congyang]

At 03/15/2012 02:46 AM, Eric Northup Wrote:
> On Wed, Mar 14, 2012 at 6:25 AM, Gleb Natapov <address@hidden> wrote:
> 
>> On Wed, Mar 14, 2012 at 03:16:05PM +0200, Avi Kivity wrote:
>>> On 03/14/2012 03:14 PM, Gleb Natapov wrote:
>>>> On Wed, Mar 14, 2012 at 03:07:46PM +0200, Avi Kivity wrote:
>>>>> On 03/14/2012 01:11 PM, Wen Congyang wrote:
>>>>>>>
>>>>>>> I don't think we want to use the driver.  Instead, have a small
>> piece of
>>>>>>> code that resets the device and pushes out a string (the panic
>> message?)
>>>>>>> without any interrupts etc.
>>>>>>>
>>>>>>> It's still going to be less reliable than a hypercall, I agree.
>>>>>>
>>>>>> Do you still want to use complicated and less reliable way?
>>>>>
>>>>> Are you willing to try it out and see how complicated it really is?
>>>>>
>>>>> While it's more complicated, it's also more flexible.  You can
>>>>> communicate the panic message, whether the guest is attempting a
>> kdump
>>>>> and its own recovery or whether it wants the host to do it, etc., you
>>>>> can communicate less severe failures like oopses.
>>>>>
>>>> hypercall can take arguments to achieve the same.
>>>
>>> It has to be designed in advance; and every time we notice something's
>>> missing we have to update the host kernel.
>>>
>>
>> We and in the designed stage now. Not to late to design something flexible
>> :) Panic hypercall can take GPA of a buffer where host puts panic info
>> as a parameter.  This buffer can be read by QEMU and passed to management.
>>
> 
> If a host kernel change is in the works, I think it might be cleanest to
> have the host kernel export a new kind of VCPU exit for unhandled-by-KVM
> hypercalls.  Then usermode can respond to the hypercall as appropriate.
>  This would permit adding or changing future hypercalls without host kernel
> changes.
> 
> "Guest panic" is almost the definition of not-a-fast-path, and so what's
> the reason to handle it in the host kernel.
> 
> Punting to user-space wouldn't be a magic bullet for getting good
> interfaces designed, but in my opinion it is a better place to be doing
> them.
> 

Do you mean that: the guest execute vmcall instruction, and the host kernel
exits to userspace. The userspace will deal with the vmexit?

Thanks
Wen Congyang