linux-kernel - Re: [PATCH v2 05/14] KVM: x86: Override reported SME/SEV feature flags with host mask

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Date:   Thu, 28 Jan 2021 16:07:31 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v2 05/14] KVM: x86: Override reported SME/SEV feature
 flags with host mask

On 14/01/21 01:36, Sean Christopherson wrote:
> Add a reverse-CPUID entry for the memory encryption word, 0x8000001F.EAX,
> and use it to override the supported CPUID flags reported to userspace.
> Masking the reported CPUID flags avoids over-reporting KVM support, e.g.
> without the mask a SEV-SNP capable CPU may incorrectly advertise SNP
> support to userspace.
> 
> Cc: Brijesh Singh <brijesh.singh@....com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/cpuid.c | 2 ++
>   arch/x86/kvm/cpuid.h | 1 +
>   2 files changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 13036cf0b912..b7618cdd06b5 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -855,6 +855,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   	case 0x8000001F:
>   		if (!boot_cpu_has(X86_FEATURE_SEV))
>   			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
> +		else
> +			cpuid_entry_override(entry, CPUID_8000_001F_EAX);
>   		break;
>   	/*Add support for Centaur's CPUID instruction*/
>   	case 0xC0000000:
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index dc921d76e42e..8b6fc9bde248 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -63,6 +63,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
>   	[CPUID_8000_0007_EBX] = {0x80000007, 0, CPUID_EBX},
>   	[CPUID_7_EDX]         = {         7, 0, CPUID_EDX},
>   	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
> +	[CPUID_8000_001F_EAX] = {0x8000001f, 1, CPUID_EAX},
>   };
>   
>   /*
> 

I don't understand, wouldn't this also need a kvm_cpu_cap_mask call 
somewhere else?  As it is, it doesn't do anything.

Paolo