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

From: Ben Warren <ben@skyportsystems.com>
To: "Marc-André Lureau" <marcandre.lureau@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	qemu-devel@nongnu.org, Gal Hammer <ghammer@redhat.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	Laszlo Ersek <lersek@redhat.com>,
	Igor Mammedov <imammedo@redhat.com>
Subject: Re: [Qemu-devel] [PULL 02/15] docs: VM Generation ID device description
Date: Wed, 12 Apr 2017 13:25:42 -0700	[thread overview]
Message-ID: <9CD33C23-31A4-408C-883C-FD9160CB4A9D@skyportsystems.com> (raw)
In-Reply-To: <CAJ+F1CLyBwaCrL=HwDJLRc3BsRX1hDQKdfv44ZFsPVjf4UoFWw@mail.gmail.com>


> On Apr 12, 2017, at 1:22 PM, Marc-André Lureau <marcandre.lureau@gmail.com> wrote:
> 
> Hi
> 
> On Thu, Apr 13, 2017 at 12:17 AM Ben Warren <ben@skyportsystems.com <mailto:ben@skyportsystems.com>> wrote:
>> On Apr 12, 2017, at 1:06 PM, Marc-André Lureau <marcandre.lureau@gmail.com <mailto:marcandre.lureau@gmail.com>> 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