linux-kernel - Re: [PATCH v15 05/41] KVM: x86: Report XSS as to-be-saved if there are supported features

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Message-ID: <28a6fa6f-26ab-458d-9f63-547d51dd6aa9@linux.intel.com>
Date: Tue, 16 Sep 2025 15:12:37 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Tom Lendacky <thomas.lendacky@....com>,
 Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>,
 Maxim Levitsky <mlevitsk@...hat.com>, Xiaoyao Li <xiaoyao.li@...el.com>,
 Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v15 05/41] KVM: x86: Report XSS as to-be-saved if there
 are supported features



On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> 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: Yang Weijiang <weijiang.yang@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Reviewed-by: Chao Gao <chao.gao@...el.com>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Binbin Wu <binbin.wu@...ux.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 771b7c883c66..3b4258b38ad8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -332,7 +332,7 @@ 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[] = {
> @@ -7499,6 +7499,10 @@ 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;
>   	}