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

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Date:   Thu, 10 Sep 2020 08:38:54 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Peter Maydell <peter.maydell@...aro.org>
Cc:     Peter Maydell <Peter.Maydell@....com>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Juan Quintela <quintela@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Richard Henderson <richard.henderson@...aro.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Steven Price <steven.price@....com>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v2 2/2] arm64: kvm: Introduce MTE VCPU feature

On Wed, Sep 09, 2020 at 04:53:02PM +0100, Peter Maydell wrote:
> On Wed, 9 Sep 2020 at 16:48, Andrew Jones <drjones@...hat.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.
>

That's a great proposal. I'll try to find time to send the patches.

Thanks,
drew