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Date: Wed, 12 Jun 2024 14:12:12 -0700
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Subject: Re: [PATCH v2 04/25] KVM: x86: Mark CR4.FRED as not reserved
From: Sean Christopherson <seanjc@google.com>
To: Xin Li <xin3.li@intel.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, 
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, 
	pbonzini@redhat.com, corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com, 
	bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, 
	shuah@kernel.org, vkuznets@redhat.com, peterz@infradead.org, 
	ravi.v.shankar@intel.com, xin@zytor.com
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On Wed, Feb 07, 2024, Xin Li wrote:
> The CR4.FRED bit, i.e., CR4[32], is no longer a reserved bit when a guest
> enumerates FRED, otherwise it is still a reserved bit.

This isn't quite correct, as __cr4_reserved_bits() is used with kvm_cpu_caps too,
i.e. to compute CR4 bits that are reserved from the host's perspective.  And that
matters, because if this check was done _only_ on guest CPUID, then KVM would
allow CR4.FRED=1 before all of KVM support is in place.

> Signed-off-by: Xin Li <xin3.li@intel.com>
> Tested-by: Shan Kang <shan.kang@intel.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  arch/x86/kvm/x86.h              | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b5b2d0fde579..0d88873eba63 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -134,7 +134,7 @@
>  			  | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
>  			  | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
>  			  | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
> -			  | X86_CR4_LAM_SUP))
> +			  | X86_CR4_LAM_SUP | X86_CR4_FRED))
>  
>  #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
>  
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 2f7e19166658..9a52016ebf5a 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -532,6 +532,8 @@ bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type);
>  		__reserved_bits |= X86_CR4_PCIDE;       \
>  	if (!__cpu_has(__c, X86_FEATURE_LAM))           \
>  		__reserved_bits |= X86_CR4_LAM_SUP;     \
> +	if (!__cpu_has(__c, X86_FEATURE_FRED))          \
> +		__reserved_bits |= X86_CR4_FRED;        \
>  	__reserved_bits;                                \
>  })
>  
> -- 
> 2.43.0
>