On Tue, Jul 30, 2024 at 07:46:12PM +0100, Daniel P. Berrangé wrote:
On Tue, Jul 30, 2024 at 02:13:51PM -0400, Peter Xu wrote:
On Mon, Jul 29, 2024 at 06:26:41PM +0100, Daniel P. Berrangé wrote:
On Mon, Jul 29, 2024 at 01:00:30PM -0400, Peter Xu wrote:
On Mon, Jul 29, 2024 at 04:58:03PM +0100, Daniel P. Berrangé wrote:
We've got two mutually conflicting goals with the machine type definitions.
Primarily we use them to ensure stable ABI, but an important secondary goal is to enable new tunables to have new defaults set, without having to update every mgmt app. The latter works very well when the defaults have no dependancy on the platform kernel/OS, but breaks migration when they do have a platform dependancy.
- Firstly, never quietly flipping any bit that affects the ABI...
- Have a default value of off, then QEMU will always allow the VM to boot by default, while advanced users can opt-in on new features. We can't make this ON by default otherwise some VMs can already fail to boot,
- If the host doesn't support the feature while the cmdline enabled it, it needs to fail QEMU boot rather than flipping, so that it says "hey, this host does not support running such VM specified, due to XXX feature missing".
That's the only way an user could understand what happened, and IMHO that's a clean way that we stick with QEMU cmdline on defining the guest ABI, while in which the machine type is the fundation of such definition, as the machine type can decides many of the rest compat properties. And that's the whole point of the compat properties too (to make sure the guest ABI is stable).
If kernel breaks it easily, all compat property things that we maintain can already stop making sense in general, because it didn't define the whole guest ABI..
So AFAIU that's really what we used for years, I hope I didn't overlook somehting. And maybe we don't yet need the "-platform" layer if we can keep up with this rule?
We've failed at this for years wrt enabling use of new defaults that have a platform depedancy, so historical practice isn't a good reference.
There are 100's (possibly 1000's) of tunables set implicitly as part of the machine type, and of those, libvirt likely only exposes a few 10's of tunables. The vast majority are low level details that no mgmt app wants to know about, they just want to accept QEMU's new defaults, while preserving machine ABI. This is a good thing. No one wants the burden of wiring up every single tunable into libvirt and mgmt apps.
This is what the "-platform" concept would be intended to preserve. It would allow a way to enable groups of settings that have a platform level dependancy, without ever having to teach either libvirt or the mgmt apps about the individual tunables.
Do you think we can achieve similar goal by simply turning the feature to ON only after a few QEMU releases? I also mentioned that idea below.
https://lore.kernel.org/r/ZqQNKZ9_OPhDq2AK@x1n
So far it really sounds like the right thing to do to me to fix all similar issues, even without introducing anything new we need to maintain.
Turning a feature with a platform dependency to "on" implies that the machine type will cease to work out of the box for platforms which lack the feature. IMHO that's not acceptable behaviour for any of our supported platforms.
Right, that's why I was thinking whether we should just always be on the safe side, even if I just replied in the other email to Akihiko, that we do have the option to make this more aggresive by turning those to ON after even 1-2 years or even less.. and we have control of how aggressive this can be.
IOW, "after a few QEMU releases" implies a delay of as much as 5 years, while we wait for platforms which don't support the feature to drop out of our supported targets list. I don't think that'll satisfy the desire to get the new feature available to users as soon as practical for their particular platform.
The feature is always available since the 1st day, right? We just need the user to opt-in, by specifying ON in the cmdline.
That'll be my take on this that QEMU's default VM setup should be always bootable, migratable, and so on. Then user opt-in on stuff like this one, where there's implication on the ABIs. The "user" can also include Libvirt. I mean when something is really important, Libvirt should, IMHO, opt-in by treating that similarly like many cpu properties, and by probing the host first.
IIUC there aren't a lot of things like that (part of guest ABI & host kernel / HW dependent), am I right? Otherwise I would expect more failures like this one, but it isn't as much as that yet. IIUC it means the efforts to make Libvirt get involved should be hopefully under control too. The worst case is Libvirt doesn't auto-on it, but again the user should always have the option to turn it on when it's necessary.
If it is left to libvirt, then it would very likely end up being a user opt-in, not auto-enabled.
Not sure whether there's other opinions, but that's definitely fine by me. I think it even makes more sense, as even if Libvirt probed the host and auto-on the feature, it also means Libvirt made a decision for the user, saying "having a better performance" is more important than "being able to migrate this VM everywhere". I don't see a way that can make such fair decision besides requesting the user to opt-in always for those, then the user is fully aware what is enabled, with the hope that when a migration fails later with "target host doesn't support feature XXX" the user is crystal clear on what happened. Thanks, -- Peter Xu