Re: [PATCH v2 2/2] arm64: kvm: Introduce MTE VCPU feature

From: Andrew Jones
Date: Thu Sep 10 2020 - 07:54:36 EST

On Thu, Sep 10, 2020 at 10:21:07AM +0100, Steven Price wrote:
> > We either need a KVM cap or a new CPU feature probing interface to avoid
> > making userspace try features one at a time. It's too bad that VCPU_INIT
> > doesn't clear all offending features from the feature set when returning
> > EINVAL, because then userspace could create a scratch VCPU with everything
> > it supports in order to see what KVM also supports in one go.
> If Peter's TELL_ME_WHAT_YOU_HAVE idea works out then perhaps we don't need
> the cap? Or would it still be useful?

We wouldn't need it, but we don't _need_ it now either. It's not very
convenient to probe vcpu features with scratch vcpus, especially if we
must probe one at a time, but it works. The TELL_ME_WHAT_YOU_HAVE idea
will only fix the one at a time issue, but still require a vcpu fd. If
this feature becomes a VM feature then a cap or VM level API would help
reduce the userspace probing work.