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@xxxxxxxxx> >Tested-by: Shan Kang <shan.kang@xxxxxxxxx> >--- > >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) ..
