linux-kernel - Re: [PATCH v2 64/66] KVM: nSVM: Expose SVM features to L1 iff nested is enabled

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Date:   Tue, 3 Mar 2020 10:37:14 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v2 64/66] KVM: nSVM: Expose SVM features to L1 iff nested
 is enabled

On Mon, Mar 2, 2020 at 3:57 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Set SVM feature bits in KVM capabilities if and only if nested=true, KVM
> shouldn't advertise features that realistically can't be used.  Use
> kvm_cpu_cap_has(X86_FEATURE_SVM) to indirectly query "nested" in
> svm_set_supported_cpuid() in anticipation of moving CPUID 0x8000000A
> adjustments into common x86 code.

Why not go ahead and report the SVM feature bits regardless of
"nested," and lock SVM off in VM_CR when nested=false? That would be
more like hardware behavior.