Re: [libvirt] [PATCH v2 06/15] qemu: Defer capability check to command line generation time

On Fri, Feb 16, 2018 at 17:28:03 +0100, Andrea Bolognani wrote:

Validate time is a bit too early to check whether the required capabilities are available, since the QEMU binary might have been updated or replaced by the time we are asked to run the guest.

So are you having problem with the fact that the definition will be rejected right away and not just when you try to start it? Validate is re-run when starting the VM so a downgrade is handled properly.