On Fri, Nov 18, 2022 at 10:18:59 +0000, John Levon wrote:

On Fri, Nov 18, 2022 at 10:52:32AM +0100, Jiri Denemark wrote:

  * Qemu already provide an option 'enforce' to validate if features
  with which vm is started is exactly same as one provided and nothing
  is silently dropped.

Right, but it's not enough. In addition to removed features libvirt also
checks for unexpectedly added features. And you really need to do both.
Because if you ask for -cpu Model,feat1=on,feat2=on,enforce and QEMU
says everything is fine, the guest might see more than what you asked.
For example, if a feature is enabled only if a host supports it you may
or may not get it without any complains from QEMU. But if you get it you
really need to explicitly ask for it during migration, otherwise the
feature can just silently disappear. Of course, this would be a really
bad behavior from QEMU, but that does not mean it can't happen (I think
SVM is a bit problematic in this way) and the whole point of libvirt's
checks is to prevent this kind of issues.

Hi Jiri, I'm not following this very well. I think you're saying that qemu has
had bugs previously where features get silently enabled,

Personally, I haven't actually witnessed any bug in this area (as far as
I can remember, which is not that far :-)), but got some reports of at
least one, even though without any proof.

Specifically, SVM is quite strange as it is included in all AMD CPU
models in QEMU and yet if you try to start it on a host without nesting
enabled "enforce" does not complain. I saw the feature is enabled with
older machine types, but I was told the magic behind this feature looks
like not only machine type but even host configuration itself is
involved. Anyway, although I saw reports of that the feature could be
enabled without an explicit request even with new machine types I still
haven't seen any proof of this happening. So I hope it just does not
happen and users are only afraid of this possibility.

and it's libvirt's job/role to paper over those issues? Do you have
some specific cases of this?

This is not about papering. It's actually the opposite, that is about
detecting if something like this happens and reporting it as a failure
rather than papering it and hoping everything goes well. So I think
doing this is a good idea even though I don't think we actually saw any
issue of this kind.

Hi Jiri, I see now with libvirt master, with check=='full' we verify both silently dropped as well as added features. But as you already stated Qemu silently adding feature is a Qemu bug and libvirt just reports that bug, so it should be very unlikely, i agree that is not a good reasoning :). Our requirement is that we want to use CPU Models and features which are defined in Qemu but not in libvirt for e.g if we want to use Icelake-Server-V4 directly or newly added vmx feature, libvirt does not allow. Currently we take help of qemu to do validations but for cpu feature verfication and model definations we still use libvirt defined definations which prevent us to use anything which is not defined in libvirt. I see there are already efforts going on to get all model and feature defination from qemu itself, but not sure how much time it will take. Till that happens we thought safest option is to have an option to remove all validations from libvirt and rely on qemu 'enforce' for migration safetly. I understand qemu-enforce does not check for silently added features, but that case we can assume is very unlikely and Qemu should fix otherwise VMs will not poweron anyway with check=='full'. Basically we want it as an modification of check='none' but also skipping things like virCPUValidateFeatures and passing option 'enforce' to Qemu. Or if silently adding features is that big concern we can have a check in Qemu itself? I understand current qemu-enforce is not as migration safe as check=='full' but probably suitable for our use case for time being?

Thanks

Manish Mishra

Jirka