Re: [PATCH v10 19/28] KVM: x86: Advertise support for FRED

[Binbin Wu]

On 9/12/2026 5:36 AM, Sohil Mehta wrote:
> From: "Xin Li (Intel)" <xin@xxxxxxxxx>
> 
> Advertise support for FRED to userspace after changes required to enable
> FRED in a KVM guest are in place.
> 
> Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>

The code change itself looks good to me.
But this patch should be moved after patch 25.

X86_CR4_FRED is added to nested.msrs.cr4_fixed1 in patch 25, before that
if the L1 guest enables FRED, it will break the nested case when do VMXON,
which checks nested_host_cr4_valid() and it will fail during the bisect
window before patch 25.

> ---
> v10:
>  - kvm_cpu_cap_clear(X86_FEATURE_FRED) in svm_set_cpu_caps() (Chao).
> ---
>  arch/x86/kvm/cpuid.c   | 1 +
>  arch/x86/kvm/svm/svm.c | 3 +++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index ddb022cb203a..5682bacdfd32 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1034,6 +1034,7 @@ void kvm_initialize_cpu_caps(void)
>  		F(FSRS),
>  		F(FSRC),
>  		F(WRMSRNS),
> +		X86_64_F(FRED),
>  		X86_64_F(LKGS),
>  		F(AMX_FP16),
>  		F(AVX_IFMA),
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f254a5ff570d..dd19c7b4e904 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5518,6 +5518,9 @@ static __init void svm_set_cpu_caps(void)
>  
>  	kvm_cpu_cap_clear(X86_FEATURE_IBT);
>  
> +	/* SVM FRED virtualization not implemented yet */
> +	kvm_cpu_cap_clear(X86_FEATURE_FRED);
> +
>  	/* CPUID 0x80000001 and 0x8000000A (SVM features) */
>  	if (nested) {
>  		kvm_cpu_cap_set(X86_FEATURE_SVM);