Re: [Xen-devel] [PATCH 03/28] libxl: Provide libxl__dm_support_*

[Jim Fehlig <jfehlig@xxxxxxxx>]

Ian Campbell wrote:
> On Mon, 2016-01-11 at 10:00 -0700, Jim Fehlig wrote:
>> On 01/07/2016 10:13 AM, Ian Campbell wrote:
>>> On Tue, 2015-12-22 at 18:44 +0000, Ian Jackson wrote:
>>>> This allows code elsewhere in libxl to find out what options a device
>>>> model executable supports.  This is done by searching the usage
>>>> message for fixed strings.
>>> Has anyone (ever, not necessarily a Xen person nor in this context)
>>> approached upstream QEMU about a machine readable output of some sort?
>>>
>>> I know libvirt does something similar to this, but they want to support
>>> older versions, whereas we at least have the luxury of not caring about
>>> versions before the point this code lands.
>> Since qemu 1.2.0, libvirt has been using the various QMP commands to probe 
>> for
>> qemu capabilities, instead of parsing help output.
> 
> As in it spawns a qemu specifically to ask the questions and then kills it
> and starts what it needs _or_ it starts the qemu with minimal command line
> cfg and then dynamically pokes in the full config via qmp?

The latter. When the qemu driver loads, it starts qemu, pokes it for
capabilities via qmp, caches what it finds, then terminates it. The cached
capabilities are used until the associated qemu binary is changed/updated.

If interested in peeking at the code, see virQEMUCapsInitQMP() and the functions
it calls in $libvirt_src/src/qemu/qemu_capabilities.c. E.g.

virQEMUCapsInitQMP()
  -> virQEMUCapsInitQMPMonitor()
    -> virQEMUCapsInitQMPBasic()

Regards,
Jim


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel