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

[Wen Congyang]

At 03/14/2012 06:52 PM, Gleb Natapov Wrote:
> On Wed, Mar 14, 2012 at 06:52:07PM +0800, Wen Congyang wrote:
>> 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.
>>
> Why? Root user can also call panic hypercall if he wishes so.

IIRC, the instruction vmcall needs to run on ring0. The root user is in ring3.

Thanks
Wen Congyang

> 
> --
>                       Gleb.
>