Re: [PATCH v13 04/21] KVM: x86: Initialize kvm_caps.supported_xss

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/21/2025 9:30 PM, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>
> 
> Set original kvm_caps.supported_xss to (host_xss & KVM_SUPPORTED_XSS) if
> XSAVES is supported. host_xss contains the host supported xstate feature
> bits for thread FPU context switch, KVM_SUPPORTED_XSS includes all KVM
> enabled XSS feature bits, the resulting value represents the supervisor
> xstates that are available to guest and are backed by host FPU framework
> for swapping {guest,host} XSAVE-managed registers/MSRs.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Reviewed-by: Chao Gao <chao.gao@intel.com>
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
>   arch/x86/kvm/x86.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 75b7a29721bb..6b01c6e9330e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -220,6 +220,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
>   				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
>   				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
>   
> +#define KVM_SUPPORTED_XSS     0
> +
>   bool __read_mostly allow_smaller_maxphyaddr = 0;
>   EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);
>   
> @@ -9793,14 +9795,17 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>   		kvm_host.xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
>   		kvm_caps.supported_xcr0 = kvm_host.xcr0 & KVM_SUPPORTED_XCR0;
>   	}
> +
> +	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> +		rdmsrq(MSR_IA32_XSS, kvm_host.xss);
> +		kvm_caps.supported_xss = kvm_host.xss & KVM_SUPPORTED_XSS;
> +	}

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

Btw, since we now have KVM_SUPPORTED_XSS to cap the supported bits, it 
seems we can remove the

	kvm_caps.supported_xss = 0;

in both vmx_set_cpu_caps() and svm_set_cpu_caps().

>   	kvm_caps.supported_quirks = KVM_X86_VALID_QUIRKS;
>   	kvm_caps.inapplicable_quirks = KVM_X86_CONDITIONAL_QUIRKS;
>   
>   	rdmsrq_safe(MSR_EFER, &kvm_host.efer);
>   
> -	if (boot_cpu_has(X86_FEATURE_XSAVES))
> -		rdmsrq(MSR_IA32_XSS, kvm_host.xss);
> -
>   	kvm_init_pmu_capability(ops->pmu_ops);
>   
>   	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))