On Thu, Aug 01, 2024 at 01:51:00AM -0400, Michael S. Tsirkin wrote:
On Wed, Jul 31, 2024 at 08:57:52AM -0400, Peter Xu wrote:
Could you elaborate why it would fail if with what I proposed?
First I think I was wrong I misunderstood what you said. To summarise, you said:
- any new feature depending on another package is off by default - starting qemu on destination with feature enabled will fail thus migration is not started
My comment is that this "started" is from qemu point of view, from user's POV starting qemu on destination is just the 1st step of migration.
However I agree, this is better since we do not waste bandwidth, and I was wrong to say we do.
My other comment is that adding features becomes even more work than it is now.
So I suggest a single command that dumps some description of host features, to be passed to qemu on destination. qemu then fails to start on destination if some of these do not work. The advantage is that this also helps things like -cpu host, and a bunch of other things like vdpa where we like to pass through config from kernel.
The disadvantage is that it does not exactly *fix* migration, it just does not let you start it.
This feels like only half a solution, and not the most helpful half. It prevents you accidentally migrating to a host that lacks some features, but doesn't help with starting a VM that has migrate compatible features in the first place. From a user POV, the latter is what's most important. Checking for incompatible features is just a safety net that you should never need to hit, if QEMU was configured suitably to start with. So to ensure a QEMU is started with migration compatible features will still require teaching libvirt about every single feature that has a host kernel dependancy, so libvirt (or the app using libvirt) knows to turn this off. This is alot more work for both libvirt & the mgmt app, than having QEMU provide the generic "platforms" concept which is extensible without needing further work outside QEMU. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|