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

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Message-ID: <871rba8wjj.fsf@vitty.brq.redhat.com>
Date:   Fri, 16 Apr 2021 10:26:40 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Vineeth Pillai <viremana@...ux.microsoft.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>
Subject: Re: [PATCH v2 1/7] hyperv: Detect Nested virtualization support for
 SVM

Vineeth Pillai <viremana@...ux.microsoft.com> writes:

> Detect nested features exposed by Hyper-V if SVM is enabled.
>

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.


> 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 3546d3e21787..c6f812851e37 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",
> +			ms_hyperv.nested_features);
>  	}
>  
>  	/*

With the commit message expanded,

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly