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Date:   Thu, 10 Sep 2020 13:49:43 +0200
From:   Andrew Jones <drjones@redhat.com>
To:     Steven Price <steven.price@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Peter Maydell <Peter.Maydell@arm.com>, qemu-devel@nongnu.org,
        Dave Martin <Dave.Martin@arm.com>,
        Juan Quintela <quintela@redhat.com>,
        Richard Henderson <richard.henderson@linaro.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 2/2] arm64: kvm: Introduce MTE VCPU feature
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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.

Thanks,
drew