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Subject: Re: [PATCH v2 1/7] hyperv: Detect Nested virtualization support for
 SVM
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "K. Y. Srinivasan" <kys@microsoft.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hyperv@vger.kernel.org,
        Lan Tianyu <Tianyu.Lan@microsoft.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, Wei Liu <wei.liu@kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        viremana@linux.microsoft.com
References: <cover.1618492553.git.viremana@linux.microsoft.com>
 <9d12558549bc0c6f179b26f5b16c751bdfab3f74.1618492553.git.viremana@linux.microsoft.com>
 <871rba8wjj.fsf@vitty.brq.redhat.com>
From:   Vineeth Pillai <viremana@linux.microsoft.com>
Message-ID: <a50e0d90-d853-7195-56ce-42b85484a55e@linux.microsoft.com>
Date:   Fri, 16 Apr 2021 12:10:26 -0400
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> It may make sense to expand this a bit as it is probably unclear how the
> change is related to SVM.
>
> Something like:
>
> HYPERV_CPUID_NESTED_FEATURES CPUID leaf can be present on both Intel and
> AMD Hyper-V guests. Previously, the code was using
> HV_X64_ENLIGHTENED_VMCS_RECOMMENDED feature bit to determine the
> availability of nested features leaf and this complies to TLFS:
> "Recommend a nested hypervisor using the enlightened VMCS interface.
> Also indicates that additional nested enlightenments may be available
> (see leaf 0x4000000A)". Enlightened VMCS, however, is an Intel only
> feature so the detection method doesn't work for AMD. Use
> HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS.EAX CPUID information ("The
> maximum input value for hypervisor CPUID information.") instead, this
> works for both AMD and Intel.
Thanks for the input. Will update the commit message in next revision.

Thanks,
Vineeth