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Date: Wed, 12 Jun 2024 15:13:52 -0700
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Subject: Re: [PATCH v2 11/25] KVM: x86: Add kvm_is_fred_enabled()
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:
> Add kvm_is_fred_enabled() to get if FRED is enabled on a vCPU.
> 
> Signed-off-by: Xin Li <xin3.li@intel.com>
> Tested-by: Shan Kang <shan.kang@intel.com>
> ---
> 
> Change since v1:
> * Explain why it is ok to only check CR4.FRED (Chao Gao).
> ---
>  arch/x86/kvm/kvm_cache_regs.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
> index 75eae9c4998a..1d431c703fdf 100644
> --- a/arch/x86/kvm/kvm_cache_regs.h
> +++ b/arch/x86/kvm/kvm_cache_regs.h
> @@ -187,6 +187,23 @@ 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.
> + *
> + * More details at FRED Spec 6.0 Section 4.2 Enabling in CR4.
> + */
> +static __always_inline bool kvm_is_fred_enabled(struct kvm_vcpu *vcpu)

Maybe just is_fred_enabled(), or even just is_fred()?  Most helpers in x86.h that
wrap CR4/CR0 in similar ways omit the "kvm_", partly for brevity, but also because
the check is architectural, not KVM-defined (though the state obviously comes
from KVM).

> +{
> +#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))
> -- 
> 2.43.0
>