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

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Message-ID: <9a628729-1b4f-4982-a3e6-b9269c91b3c2@linux.intel.com>
Date: Wed, 21 Jan 2026 16:05:13 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 linux-doc@...r.kernel.org, seanjc@...gle.com, pbonzini@...hat.com,
 corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, luto@...nel.org,
 peterz@...radead.org, andrew.cooper3@...rix.com, chao.gao@...el.com,
 hch@...radead.org, sohil.mehta@...el.com
Subject: Re: [PATCH v9 11/22] KVM: x86: Add a helper to detect if FRED is
 enabled for a vCPU



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

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@...el.com>
> [ Sean: removed the "kvm_" prefix from the function name ]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Xin Li (Intel) <xin@...or.com>
> Tested-by: Shan Kang <shan.kang@...el.com>
> Tested-by: Xuelian Guo <xuelian.guo@...el.com>
> ---
> 
> 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))