Re: [PATCH v13 02/21] KVM: x86: Report XSS as to-be-saved if there are supported features

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

On 8/21/2025 9:30 PM, Chao Gao wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> Add MSR_IA32_XSS to list of MSRs reported to userspace if supported_xss
> is non-zero, i.e. KVM supports at least one XSS based feature.
> 
> Before enabling CET virtualization series, guest IA32_MSR_XSS is
> guaranteed to be 0, i.e., XSAVES/XRSTORS is executed in non-root mode
> with XSS == 0, which equals to the effect of XSAVE/XRSTOR.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> 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>

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

> ---
>   arch/x86/kvm/x86.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 31a7e7ad310a..569583943779 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -335,7 +335,7 @@ static const u32 msrs_to_save_base[] = {
>   	MSR_IA32_RTIT_ADDR3_A, MSR_IA32_RTIT_ADDR3_B,
>   	MSR_IA32_UMWAIT_CONTROL,
>   
> -	MSR_IA32_XFD, MSR_IA32_XFD_ERR,
> +	MSR_IA32_XFD, MSR_IA32_XFD_ERR, MSR_IA32_XSS,
>   };
>   
>   static const u32 msrs_to_save_pmu[] = {
> @@ -7474,6 +7474,10 @@ static void kvm_probe_msr_to_save(u32 msr_index)
>   		if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
>   			return;
>   		break;
> +	case MSR_IA32_XSS:
> +		if (!kvm_caps.supported_xss)
> +			return;
> +		break;
>   	default:
>   		break;
>   	}