Re: [PATCH v10 16/28] KVM: x86: Mark CR4.FRED as not reserved

[Binbin Wu]

On 9/12/2026 5:36 AM, Sohil Mehta wrote:
> From: "Xin Li (Intel)" <xin@xxxxxxxxx>
> 
> The CR4.FRED bit, i.e., CR4[32], is no longer a reserved bit when
> guest cpu cap has FRED, i.e.,
>   1) All of FRED KVM support is in place.
>   2) Guest enumerates FRED.
> 
> Otherwise it is still a reserved bit.
> 
> Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
> ---
> v10:
>  - Check CR4.FRED against IA-32e mode in kvm_set_cr4() instead of
>    __kvm_is_valid_cr4(), so that KVM_SET_SREGS no longer rejects a CR4
>    and EFER pair that are loaded together (e.g., when restoring a FRED
>    guest)
> ---
>  arch/x86/kvm/regs.c | 7 +++++++
>  arch/x86/kvm/regs.h | 4 +++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/regs.c b/arch/x86/kvm/regs.c
> index 8f66438989e4..ba6c8856b4ff 100644
> --- a/arch/x86/kvm/regs.c
> +++ b/arch/x86/kvm/regs.c
> @@ -429,6 +429,13 @@ int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  			return 1;
>  	}
>  
> +	/*
> +	 * FRED can not be enabled when EFER.LMA=0.  Note, MOV to CR4 outside
> +	 * 64-bit mode clears CR4[63:32] so only emulation can get here.
> +	 */

Could you elaborate what "emulation" means here?

> +	if ((cr4 & X86_CR4_FRED) && !is_long_mode(vcpu))
> +		return 1;
> +
>  	if ((cr4 & X86_CR4_CET) && !kvm_is_cr0_bit_set(vcpu, X86_CR0_WP))
>  		return 1;
>  
> diff --git a/arch/x86/kvm/regs.h b/arch/x86/kvm/regs.h
> index ef46918daf43..5db01693ad07 100644
> --- a/arch/x86/kvm/regs.h
> +++ b/arch/x86/kvm/regs.h
> @@ -28,7 +28,7 @@ static_assert(!(KVM_POSSIBLE_CR0_GUEST_BITS & X86_CR0_PDPTR_BITS));
>  			  | 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_CET))
> +			  | X86_CR4_LAM_SUP | X86_CR4_CET | X86_CR4_FRED))
>  
>  #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
>  
> @@ -423,6 +423,8 @@ static inline bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  	if (!__cpu_has(__c, X86_FEATURE_SHSTK) &&       \
>  	    !__cpu_has(__c, X86_FEATURE_IBT))           \
>  		__reserved_bits |= X86_CR4_CET;         \
> +	if (!__cpu_has(__c, X86_FEATURE_FRED))          \
> +		__reserved_bits |= X86_CR4_FRED;        \
>  	__reserved_bits;                                \
>  })
>