linux-kernel - Re: [PATCH v9 10/22] KVM: VMX: Add support for saving and restoring FRED MSRs

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Message-ID: <aRQmRZYQr579kT4N@intel.com>
Date: Wed, 12 Nov 2025 14:16:37 +0800
From: Chao Gao <chao.gao@...el.com>
To: "Xin Li (Intel)" <xin@...or.com>
CC: <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <pbonzini@...hat.com>, <seanjc@...gle.com>,
	<corbet@....net>, <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>,
	<luto@...nel.org>, <peterz@...radead.org>, <andrew.cooper3@...rix.com>,
	<hch@...radead.org>, <sohil.mehta@...el.com>
Subject: Re: [PATCH v9 10/22] KVM: VMX: Add support for saving and restoring
 FRED MSRs

>@@ -4316,6 +4374,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> #endif
> 	case MSR_IA32_U_CET:
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
>+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
>+			WARN_ON_ONCE(msr != MSR_IA32_FRED_SSP0);

This will be triggered if the guest only supports IBT and tries to write U_CET here.

>+			vcpu->arch.fred_ssp0_fallback = data;
>+			break;
>+		}
>+
> 		kvm_set_xstate_msr(vcpu, msr_info);
> 		break;
> 	default:
>@@ -4669,6 +4733,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> #endif
> 	case MSR_IA32_U_CET:
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
>+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
>+			WARN_ON_ONCE(msr_info->index != MSR_IA32_FRED_SSP0);

ditto.

With this fixed,

Reviewed-by: Chao Gao <chao.gao@...el.com>