Re: [PATCH v9 11/22] KVM: x86: Add a helper to detect if FRED is enabled for a vCPU

[Binbin Wu]

On 10/27/2025 4:18 AM, Xin Li (Intel) wrote:
> From: Xin Li <xin3.li@xxxxxxxxx>
> 

Not sure if it's OK with empty change log even though the patch is simple and
the title has already described it.

> Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
> [ Sean: removed the "kvm_" prefix from the function name ]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
> Tested-by: Xuelian Guo <xuelian.guo@xxxxxxxxx>
> ---
> 
> Change in v5:
> * Add TB from Xuelian Guo.
> ---
>  arch/x86/kvm/kvm_cache_regs.h | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
> index 8ddb01191d6f..3c8dbb77d7d4 100644
> --- a/arch/x86/kvm/kvm_cache_regs.h
> +++ b/arch/x86/kvm/kvm_cache_regs.h
> @@ -205,6 +205,21 @@ static __always_inline bool kvm_is_cr4_bit_set(struct kvm_vcpu *vcpu,
>  	return !!kvm_read_cr4_bits(vcpu, cr4_bit);
>  }
>  
> +/*
> + * It's enough to check just CR4.FRED (X86_CR4_FRED) to tell if
> + * a vCPU is running with FRED enabled, because:
> + * 1) CR4.FRED can be set to 1 only _after_ IA32_EFER.LMA = 1.
> + * 2) To leave IA-32e mode, CR4.FRED must be cleared first.
> + */
> +static inline bool is_fred_enabled(struct kvm_vcpu *vcpu)
> +{
> +#ifdef CONFIG_X86_64
> +	return kvm_is_cr4_bit_set(vcpu, X86_CR4_FRED);
> +#else
> +	return false;
> +#endif
> +}
> +
>  static inline ulong kvm_read_cr3(struct kvm_vcpu *vcpu)
>  {
>  	if (!kvm_register_is_available(vcpu, VCPU_EXREG_CR3))