linux-kernel - RE: [PATCH v5 1/7] hyperv: Detect Nested virtualization support for SVM

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Date:   Tue, 8 Jun 2021 16:59:58 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Vineeth Pillai <viremana@...ux.microsoft.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        vkuznets <vkuznets@...hat.com>,
        Tom Lendacky <thomas.lendacky@....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>
CC:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        KY Srinivasan <kys@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH v5 1/7] hyperv: Detect Nested virtualization support for
 SVM

From: Vineeth Pillai <viremana@...ux.microsoft.com> Sent: Thursday, June 3, 2021 8:15 AM
> 
> Previously, to detect nested virtualization enlightenment support,
> we were using HV_X64_ENLIGHTENED_VMCS_RECOMMENDED feature bit of
> HYPERV_CPUID_ENLIGHTMENT_INFO.EAX CPUID as docuemented in TLFS:

s/docuemented/documented/

>  "Bit 14: 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 above
> detection method doesn't work for AMD. So, use the
> HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS.EAX CPUID information ("The
> maximum input value for hypervisor CPUID information.") and this
> works for both AMD and Intel.
> 
> Signed-off-by: Vineeth Pillai <viremana@...ux.microsoft.com>
> ---
>  arch/x86/kernel/cpu/mshyperv.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 22f13343b5da..c268c2730048 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -252,6 +252,7 @@ static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
> 
>  static void __init ms_hyperv_init_platform(void)
>  {
> +	int hv_max_functions_eax;
>  	int hv_host_info_eax;
>  	int hv_host_info_ebx;
>  	int hv_host_info_ecx;
> @@ -269,6 +270,8 @@ static void __init ms_hyperv_init_platform(void)
>  	ms_hyperv.misc_features = cpuid_edx(HYPERV_CPUID_FEATURES);
>  	ms_hyperv.hints    = cpuid_eax(HYPERV_CPUID_ENLIGHTMENT_INFO);
> 
> +	hv_max_functions_eax =
> cpuid_eax(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS);
> +
>  	pr_info("Hyper-V: privilege flags low 0x%x, high 0x%x, hints 0x%x, misc 0x%x\n",
>  		ms_hyperv.features, ms_hyperv.priv_high, ms_hyperv.hints,
>  		ms_hyperv.misc_features);
> @@ -298,8 +301,7 @@ static void __init ms_hyperv_init_platform(void)
>  	/*
>  	 * Extract host information.
>  	 */
> -	if (cpuid_eax(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS) >=
> -	    HYPERV_CPUID_VERSION) {
> +	if (hv_max_functions_eax >= HYPERV_CPUID_VERSION) {
>  		hv_host_info_eax = cpuid_eax(HYPERV_CPUID_VERSION);
>  		hv_host_info_ebx = cpuid_ebx(HYPERV_CPUID_VERSION);
>  		hv_host_info_ecx = cpuid_ecx(HYPERV_CPUID_VERSION);
> @@ -325,9 +327,11 @@ 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) {
> +	if (hv_max_functions_eax >= HYPERV_CPUID_NESTED_FEATURES) {
>  		ms_hyperv.nested_features =
>  			cpuid_eax(HYPERV_CPUID_NESTED_FEATURES);
> +		pr_info("Hyper-V: Nested features: 0x%x\n",

Nit:  Drop the colon after "Nested features".  Current code isn't very consistent
but I'm trying to establish the pattern of "Hyper-V:" followed by names and
values, with multiple name/value pairs separated by a comma.

> +			ms_hyperv.nested_features);
>  	}
> 
>  	/*
> --
> 2.25.1

Nits notwithstanding,

Reviewed-by: Michael Kelley <mikelley@...rosoft.com>