linux-kernel - Re: [PATCH v2 1/7] hyperv: Detect Nested virtualization support for SVM

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Message-ID: <a50e0d90-d853-7195-56ce-42b85484a55e@linux.microsoft.com>
Date:   Fri, 16 Apr 2021 12:10:26 -0400
From:   Vineeth Pillai <viremana@...ux.microsoft.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "K. Y. Srinivasan" <kys@...rosoft.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org,
        Lan Tianyu <Tianyu.Lan@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Wei Liu <wei.liu@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        viremana@...ux.microsoft.com
Subject: Re: [PATCH v2 1/7] hyperv: Detect Nested virtualization support for
 SVM


> 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