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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

> 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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