Re: [PATCH v2 11/25] KVM: x86: Add kvm_is_fred_enabled()

[Chao Gao <chao.gao@intel.com>]

On Thu, Feb 08, 2024 at 01:26:31AM +0800, 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.
>+ */

I think we can give more context here, e.g.,

Although FRED architecture applies to 64-bit mode only, there is no need to
check if the CPU is in 64-bit mode (i.e., IA32_EFER.LMA and CS.L) to tell if
FRED is enabled because CR4.FRED=1 implies the CPU is in 64-bit mode.
Specifically,

1) ..
2) ..