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

[Wen Congyang]

At 03/14/2012 06:37 PM, Amit Shah Wrote:
> On (Wed) 14 Mar 2012 [17:53:00], 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?
> 
> Depends on how you want to use it.  It could be the kernel, or it
> could be a userspace program which monitors syslogs for panic
> information and passes on that info to the virtio-serial channel.

When the kernel is panicked, we cannot use userspace program.

> 
>> 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.
> 
> Access control is via permissions.  You can have udev scripts assign
> whatever uid and gid to the port of your interest.  By default, all
> ports are only accessible to the root user.

We should also prevent root user writing message to this channel if it is
used for panicked notification.

Thanks
Wen Congyang

> 
> 		Amit
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