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From:   Peter Maydell <peter.maydell@linaro.org>
Date:   Wed, 9 Sep 2020 16:53:02 +0100
Message-ID: <CAFEAcA_1YnBg3HVmtrSMP8u7GqHY8mtDAJwVcZuNUhwA+q9q=w@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] arm64: kvm: Introduce MTE VCPU feature
To:     Andrew Jones <drjones@redhat.com>
Cc:     Steven Price <steven.price@arm.com>,
        Peter Maydell <Peter.Maydell@arm.com>,
        Juan Quintela <quintela@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Richard Henderson <richard.henderson@linaro.org>,
        QEMU Developers <qemu-devel@nongnu.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        kvmarm@lists.cs.columbia.edu,
        arm-mail-list <linux-arm-kernel@lists.infradead.org>,
        Marc Zyngier <maz@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>,
        Dave Martin <Dave.Martin@arm.com>,
        lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, 9 Sep 2020 at 16:48, Andrew Jones <drjones@redhat.com> 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.

You could add one if you wanted -- add a new feature bit
TELL_ME_WHAT_YOU_HAVE. If the kernel sees that then on filure
it clears out feature bits it doesn't support and also clears
TELL_ME_WHAT_YOU_HAVE. If QEMU sees EINVAL and TELL_ME_WHAT_YOU_HAVE
is still set, then it knows it's dealing with an old kernel
and has to do one-at-a-time probing. If it sees EINVAL but not
TELL_ME_WHAT_YOU_HAVE then it knows it has a new kernel and
has just got all the info.

-- PMM