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Re: [Qemu-devel] [PULL 02/15] docs: VM Generation ID device description


From: Ben Warren
Subject: Re: [Qemu-devel] [PULL 02/15] docs: VM Generation ID device description
Date: Wed, 12 Apr 2017 13:25:42 -0700

> On Apr 12, 2017, at 1:22 PM, Marc-André Lureau <address@hidden> wrote:
> 
> Hi
> 
> On Thu, Apr 13, 2017 at 12:17 AM Ben Warren <address@hidden 
> <mailto:address@hidden>> wrote:
>> On Apr 12, 2017, at 1:06 PM, Marc-André Lureau <address@hidden 
>> <mailto:address@hidden>> wrote:
>> 
>> +Device Usage:
>> +-------------
>> +
>> +The device has one property, which may be only be set using the command 
>> line:
>> +
>> +  guid - sets the value of the GUID.  A special value "auto" instructs
>> +         QEMU to generate a new random GUID.
>> +
>> +For example:
>> +
>> +  QEMU  -device vmgenid,guid="324e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87"
>> +  QEMU  -device vmgenid,guid=auto
>> 
>> The default will keep uuid to null, should it be documented? Wouldn't it 
>> make sense to default to auto?
> 
> There is no default - you have to supply a value. It’s up to whatever 
> software is managing VM lifecycle to decide what value to pass in.  Always 
> setting to ‘auto’ will cause a lot of churn within Windows that may or may 
> not be acceptable to your use case.
> 
> 
> Why would you have a vmgenid device if it's always null? Does that please 
> some windows use-cases as well? 
>  
I don’t get what you mean by this.  What device is always null?  Either the 
device is instantiated or it isn’t.  If not there, Windows will not find a 
device and I don’t know how derived objects (Invocation ID, etc.) are handled.
>>  
>> +The property may be queried via QMP/HMP:
>> +
>> +  (QEMU) query-vm-generation-id
>> +  {"return": {"guid": "324e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87"}}
>> +
>> +Setting of this parameter is intentionally left out from the QMP/HMP
>> +interfaces.  There are no known use cases for changing the GUID once QEMU is
>> +running, and adding this capability would greatly increase the complexity.
>>  
>> Is this supposed to be not permitted?
>> 
>> { "execute": "qom-set", "arguments": { "path": 
>> "/machine/peripheral-anon/device[1]", "property": "guid", "value": "auto" } }
>> 
>> Is there any linux kernel support being worked on?
> 
> This isn’t really relevant to the Linux kernel, at least in any way I can 
> think of.  What did you have in mind?
> 
> Testing, but apparently we do have RFE for RHEL as Laszlo pointed out.
OK, so you mean a guest driver.  I do have one that needs work to go upstream, 
but has been helpful to me in testing.
https://github.com/ben-skyportsystems/vmgenid-test 
<https://github.com/ben-skyportsystems/vmgenid-test>

> 
> Thanks
> -- 
> Marc-André Lureau



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