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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Vineeth Pillai <viremana@linux.microsoft.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>
Subject: Re: [PATCH 1/7] hyperv: Detect Nested virtualization support for SVM
In-Reply-To: <e14dac75ff1088b2c4bea361954b37e414edd03c.1617804573.git.viremana@linux.microsoft.com>
References: <cover.1617804573.git.viremana@linux.microsoft.com>
 <e14dac75ff1088b2c4bea361954b37e414edd03c.1617804573.git.viremana@linux.microsoft.com>
Date:   Thu, 08 Apr 2021 13:06:53 +0200
Message-ID: <87lf9tavci.fsf@vitty.brq.redhat.com>
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Vineeth Pillai <viremana@linux.microsoft.com> writes:

> Detect nested features exposed by Hyper-V if SVM is enabled.
>
> Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
> ---
>  arch/x86/kernel/cpu/mshyperv.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 3546d3e21787..4d364acfe95d 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -325,9 +325,17 @@ static void __init ms_hyperv_init_platform(void)
>  			ms_hyperv.isolation_config_a, ms_hyperv.isolation_config_b);
>  	}
>  
> -	if (ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {
> +	/*
> +	 * AMD does not need enlightened VMCS as VMCB is already a
> +	 * datastructure in memory. 

Well, VMCS is also a structure in memory, isn't it? It's just that we
don't have a 'clean field' concept for it and we can't use normal memory
accesses.

> 	We need to get the nested
> +	 * features if SVM is enabled.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_SVM) ||
> +	    ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {

Do I understand correctly that we can just look at CPUID.0x40000000.EAX
and in case it is >= 0x4000000A we can read HYPERV_CPUID_NESTED_FEATURES
leaf? I'd suggest we do that intead then.

>  		ms_hyperv.nested_features =
>  			cpuid_eax(HYPERV_CPUID_NESTED_FEATURES);
> +		pr_info("Hyper-V nested_features: 0x%x\n",
> +			ms_hyperv.nested_features);
>  	}
>  
>  	/*

-- 
Vitaly