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

[Wen Congyang]

At 03/14/2012 06:58 PM, Gleb Natapov Wrote:
> On Wed, Mar 14, 2012 at 06:57:59PM +0800, Wen Congyang wrote:
>> 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.
>>
> And who will stop the root from loading kernel module?

Yes, I forgot this.

Thanks
Wen Congyang

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