Re: [PATCH v7.5] kvm: notify host when the guest is panicked

[Wen Congyang <wency@cn.fujitsu.com>]

At 07/23/2012 07:50 AM, Sasha Levin Wrote:
> On 07/23/2012 12:36 AM, Anthony Liguori wrote:
>> Sasha Levin <levinsasha928@gmail.com> writes:
>>
>>> On 07/22/2012 09:14 PM, Anthony Liguori wrote:
>>>> Sasha Levin <levinsasha928@gmail.com> writes:
>>>>
>>>>> On 07/21/2012 10:44 AM, Wen Congyang wrote:
>>>>>> We can know the guest is panicked when the guest runs on xen.
>>>>>> But we do not have such feature on kvm.
>>>>>>
>>>>>> Another purpose of this feature is: management app(for example:
>>>>>> libvirt) can do auto dump when the guest is panicked. If management
>>>>>> app does not do auto dump, the guest's user can do dump by hand if
>>>>>> he sees the guest is panicked.
>>>>>>
>>>>>> We have three solutions to implement this feature:
>>>>>> 1. use vmcall
>>>>>> 2. use I/O port
>>>>>> 3. use virtio-serial.
>>>>>>
>>>>>> We have decided to avoid touching hypervisor. The reason why I choose
>>>>>> choose the I/O port is:
>>>>>> 1. it is easier to implememt
>>>>>> 2. it does not depend any virtual device
>>>>>> 3. it can work when starting the kernel
>>>>>
>>>>> Was the option of implementing a virtio-watchdog driver considered?
>>>>>
>>>>> You're basically re-implementing a watchdog, a guest-host interface and a set of protocols for guest-host communications.
>>>>>
>>>>> Why can't we re-use everything we have now, push a virtio watchdog
>>>>> driver into drivers/watchdog/, and gain a more complete solution to
>>>>> detecting hangs inside the guest.
>>>>
>>>> The purpose of virtio is not to reinvent every possible type of device.
>>>> There are plenty of hardware watchdogs that are very suitable to be used
>>>> for this purpose.  QEMU implements quite a few already.
>>>>
>>>> Watchdogs are not performance sensitive so there's no point in using
>>>> virtio.
>>>
>>> The issue here is not performance, but the adding of a brand new
>>> guest-host interface.
>>
>> We have:
>>
>> 1) Virtio--this is our preferred PV interface.  It needs PCI to be fully
>> initialized and probably will live as a module.
>>
>> 2) Hypercalls--this a secondary PV interface but is available very
>> early.  It's terminated in kvm.ko which means it can only operate on
>> things that are logically part of the CPU and/or APIC complex.
>>
>> This patch introduces a third interface which is available early like
>> hypercalls but not necessarily terminated in kvm.ko.  That means it can
>> have a broader scope in functionality than (2).
>>
>> We could just as well use a hypercall and have multiple commands issued
>> to that hypercall as a convention and add a new exit type to KVM that
>> sent that specific hypercall to userspace for processing.
>>
>> But a PIO operation already has this behavior and requires no changes to kvm.ko.
> 
> I don't dispute that there may be a need for another guest-host interface, but this patch can basically be called "kvm: notify host when the guest is panicked, oh, btw, and add a brand new undocumented interface"

I forgot to document this interface. I will add it.

Thanks
Wen Congyang

> 
> The new interface should at least come in it's own patch, with documentation.
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