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

[Wen Congyang]

At 03/14/2012 05:51 PM, Amit Shah Wrote:
> On (Wed) 14 Mar 2012 [16:29:50], 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 start vm like this:
>> qemu ...\
>>    -device virtio-serial \
>>   -chardev socket,path=/tmp/foo,server,nowait,id=foo \
>>   -device virtserialport,chardev=foo,name=port1 ...
> 
> This is sufficient.  On the host, you can open /tmp/foo using a custom
> program or nc (nc -U /tmp/foo).  On the guest, you can just open
> /dev/virtio-ports/port1 and read/write into it.

I have two questions:
1. does it OK to open this device when the guest is panicked?
2. how to prevent the userspace's program using this device?

Thanks
Wen Congyang

> 
> See the following links for more details.
> 
> https://fedoraproject.org/wiki/Features/VirtioSerial#How_To_Test
> http://www.linux-kvm.org/page/Virtio-serial_API
> 
>> You said that there are too many channels. Does it mean /tmp/foo is a channel?
> 
> You can have several such -device virtserialport.  The -device part
> describes what the guest will see.  The -chardev part ties that to the
> host-side part of the channel.
> 
> /tmp/foo is the host end-point for the channel, in the example above,
> and /dev/virtio-ports/port1 is the guest-side end-point.
> 
> 		Amit
> 

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