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

[Jan Kiszka]

On 2012-03-15 11:39, Gleb Natapov wrote:
> On Wed, Mar 14, 2012 at 11:46:08AM -0700, 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.
>>
> There was such vm exit (KVM_EXIT_HYPERCALL), but it was deemed to be a
> bad idea.

BTW, this would help a lot in emulating hypercalls of other hypervisors
(or of KVM's VAPIC in the absence of in-kernel irqchip - I had to jump
through hoops therefore) in user space. Not all those hypercall handlers
actually have to reside in the KVM module.

Jan

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