linux-kernel - Re: [PATCH v4 2/6] KVM: x86: Introduce CPUID_8000_0007

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Date:   Thu, 22 Sep 2022 10:55:49 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/6] KVM: x86: Introduce CPUID_8000_0007_EDX
 'scattered' leaf

On Thu, Sep 22, 2022 at 10:53 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Sep 22, 2022, Jim Mattson wrote:
> > Why do we need a new 'scattered' leaf? We are only using two bits in
> > the first one, right? And now we're only going to use one bit in the
> > second one?
> >
> > I thought the point of the 'scattered' leaves was to collect a bunch
> > of feature bits from different CPUID leaves in a single feature word.
> >
> > Allocating a new feature word for one or two bits seems extravagant.
>
> Ah, these leafs aren't scattered from KVM's perspective.
>
> The scattered terminology comes from the kernel side, where the KVM-only leafs
> _may_ be used to deal with features that are scattered by the kernel.  But there
> is no requirement that KVM-only leafs _must_ be scattered by the kernel, e.g. we
> can and should use this for leafs that KVM wants to expose to the guest, but are
> completely ignored by the kernel.  Intel's PSFD feature flag is a good example.
>
> A better shortlog would be:
>
>   KVM: x86: Add a KVM-only leaf for CPUID_8000_0007_EDX

Thanks. The 'scattered' terminology seems more confusing than enlightening.