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Date:   Thu, 10 Sep 2020 12:01:01 +0200
From:   Andrew Jones <drjones@redhat.com>
To:     Peter Maydell <peter.maydell@linaro.org>
Cc:     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>,
        lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Price <steven.price@arm.com>,
        Will Deacon <will@kernel.org>, kvmarm@lists.cs.columbia.edu,
        arm-mail-list <linux-arm-kernel@lists.infradead.org>,
        Dave Martin <Dave.Martin@arm.com>
Subject: Re: [PATCH v2 2/2] arm64: kvm: Introduce MTE VCPU feature
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On Thu, Sep 10, 2020 at 08:38:54AM +0200, Andrew Jones wrote:
> On Wed, Sep 09, 2020 at 04:53:02PM +0100, Peter Maydell wrote:
> > 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.
> >
> 
> That's a great proposal. I'll try to find time to send the patches.
>

We also have KVM_ARM_PREFERRED_TARGET, which is documented as

"""
...
The ioctl returns struct kvm_vcpu_init instance containing information
about preferred CPU target type and recommended features for it.  The
kvm_vcpu_init->features bitmap returned will have feature bits set if
the preferred target recommends setting these features, but this is
not mandatory.
...
"""

But, it says "recommended" features, not "all supported" features,
and the current implementation of KVM_ARM_PREFERRED_TARGET only
zeros out features. So, I think we should just leave
KVM_ARM_PREFERRED_TARGET as is and stick to the plan of extending
VCPU_INIT.

Thanks,
drew