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

[Wen Congyang]

At 03/14/2012 06:07 PM, Avi Kivity Wrote:
> On 03/14/2012 11:53 AM, Wen Congyang wrote:
>> At 03/14/2012 05:24 PM, Avi Kivity Wrote:
>>> On 03/14/2012 10:29 AM, Wen Congyang wrote:
>>>> At 03/13/2012 06:47 PM, Avi Kivity Wrote:
>>>>> On 03/13/2012 11:18 AM, Daniel P. Berrange wrote:
>>>>>> On Mon, Mar 12, 2012 at 12:33:33PM +0200, Avi Kivity wrote:
>>>>>>> On 03/12/2012 11:04 AM, Wen Congyang wrote:
>>>>>>>> Do you have any other comments about this patch?
>>>>>>>>
>>>>>>>
>>>>>>> Not really, but I'm not 100% convinced the patch is worthwhile.  It's
>>>>>>> likely to only be used by Linux, which has kexec facilities, and you can
>>>>>>> put talk to management via virtio-serial and describe the crash in more
>>>>>>> details than a simple hypercall.
>>>>>>
>>>>>> As mentioned before, I don't think virtio-serial is a good fit for this.
>>>>>> We want something that is simple & guaranteed always available. Using
>>>>>> virtio-serial requires significant setup work on both the host and guest.
>>>>>
>>>>> So what?  It needs to be done anyway for the guest agent.
>>>>>
>>>>>> Many management application won't know to make a vioserial device 
>>>>>> available
>>>>>> to all guests they create. 
>>>>>
>>>>> Then they won't know to deal with the panic event either.
>>>>>
>>>>>> Most administrators won't even configure kexec,
>>>>>> let alone virtio serial on top of it. 
>>>>>
>>>>> It should be done by the OS vendor, not the individual admin.
>>>>>
>>>>>> The hypercall requires zero host
>>>>>> side config, and zero guest side config, which IMHO is what we need for
>>>>>> this feature.
>>>>>
>>>>> If it was this one feature, yes.  But we keep getting more and more
>>>>> features like that and we bloat the hypervisor.  There's a reason we
>>>>> have a host-to-guest channel, we should use it.
>>>>>
>>>>
>>>> I donot know how to use virtio-serial.
>>>
>>> I don't either, copying Amit.
>>>
>>>> I start vm like this:
>>>> qemu ...\
>>>>    -device virtio-serial \
>>>>   -chardev socket,path=/tmp/foo,server,nowait,id=foo \
>>>>   -device virtserialport,chardev=foo,name=port1 ...
>>>>
>>>> You said that there are too many channels. Does it mean /tmp/foo is a 
>>>> channel?
>>>
>>> Probably.
>>
>> Hmm, if we use virtio-serial, the guest kernel writes something into the 
>> channel when
>> the os is panicked. Is it right?
> 
> Right.
> 
>> If so, is this channel visible to guest userspace? If the channle is visible 
>> to guest
>> userspace, the program running in userspace may write the same message to 
>> the channel.
>>
> 
> Surely there's some kind of access control on channels.

The virtio-serial depends on more things than touching the hypervisor. So I 
think touching
the hypervisor is more reliable than using virtio-serial device, and it is very 
simple and
easy to use.

If we pass something from guest userspace to host, we can use virtio-serial. 
But If we pass
something from guest kernelspace to host, I still prefer to touch the 
hypervisor.

Thanks
Wen Congyang