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Re: [libvirt] [PATCH 2/6] domain_conf: Add USB controler model "none"



On 07/10/2012 01:01 AM, Osier Yang wrote:
> On 2012年07月10日 01:29, Peter Krempa wrote:
>> Libvirt adds a USB controller to the guest even if the user does not
>> specify any in the XML. This is due to back-compat reasons.
> 
> It's not for back-compat reasons as far as I known, 42043afcd adds
> the implicit USB controller for virt-manager's use, and it causes
> the regression bug of migration, as the old libvirt doesn't have
> a default USB controller as a force. Jirka fixed it by commit
> 409b5f549.

It IS for back-compat reasons.  We have been outputting '-usb' in the
qemu command line for ages prior to 42043afcd, and need a way to
explicitly omit the '-usb' in the command line.  Older libvirt had no
way to represent the implicit device, and the fix of 409b5f549 was more
of a hack that said when migrating back to an older libvirt, the
existing usb controller of the sending libvirt would be implicitly
recreated by the receiving libvirt.

If a user explicitly requests the 'none' controller, then we must ensure
that migration to an older host fails up front; the 'none' element must
be explicit because the absence of an element implies the default usb
controller.

> 
> Is it a hard requirement of specify the 'none' USB controller
> model?

Yes, just like it is a hard requirement that a user explicitly specifies
the 'none' memoryballoon controller.  That's the back-compat issue we
have to face any time we make the XML explicit for something that older
libvirt provided implicitly.  This patch looks correct to me.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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