[libvirt] [PATCH] conf: Fix ABI stability check for spicevmc channel

[Jiri Denemark]

On Fri, Aug 17, 2012 at 06:51:22 -0600, Eric Blake wrote:
> On 08/17/2012 06:20 AM, Jiri Denemark wrote:
> > Change device type of a virtio channel from/to spicevmc is not a user
> > visible change. However, spicevmc channels use different default target
> > name than other virtio channels. To maintain ABI stability during this
> > change target name must be explicitly specified (and equal) in both
> > configurations.
> 
> Can we go from 'non-spice, no name' to 'spice, explicit name that
> matches old non-spice default name'?  Can we go from 'spice, no name' to
> 'non-spice, explicit name that matches old spice default name'?  Your
> patch prevents those, even if it is technically possible.

> For that matter, can we even go from 'non-spice, no name' to 'non-spice,
> explicit default name'?  If we can, then the user can achieve the
> earlier task through two steps.

In theory, both can be done but both cases are forbidden by ABI stability
checks. The main reason is that ABI stability check is a generic code which
has no idea what the default values are. Moving the defaults from qemu driver
to conf is certainly doable but I think we should avoid doing so unless we
absolutely have to.

Jirka