linux-kernel - Re: [PATCH v5 10/23] KVM: VMX: Add support for FRED context save/restore

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Message-ID: <aIHGFpKfkyDisYaZ@intel.com>
Date: Thu, 24 Jul 2025 13:35:18 +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>
Subject: Re: [PATCH v5 10/23] KVM: VMX: Add support for FRED context
 save/restore

>@@ -1870,6 +1873,37 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
> 
> 		data = (u32)data;
> 		break;
>+	case MSR_IA32_FRED_STKLVLS:
>+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
>+			return 1;

Return KVM_MSR_RET_UNSUPPORTED instead of 1.

KVM's uAPI allows userspace to read MSRs and write 0 to MSRs even if an MSR
isn't supported according to guest CPUIDs. Returning KVM_MSR_RET_UNSUPPORTED
allows kvm_do_msr_access() to suppress the failure when needed to comply with
the uAPI.

For more details, see kvm_do_msr_access().

>+		break;
>+	case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_RSP3:
>+	case MSR_IA32_FRED_SSP1 ... MSR_IA32_FRED_CONFIG:
>+		u64 reserved_bits = 0;
>+
>+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
>+			return 1;

Ditto here and for __kvm_get_msr() below.

>+
>+		if (is_noncanonical_msr_address(data, vcpu))
>+			return 1;
>+
>+		switch (index) {
>+		case MSR_IA32_FRED_CONFIG:
>+			reserved_bits = BIT_ULL(11) | GENMASK_ULL(5, 4) | BIT_ULL(2);
>+			break;
>+		case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_RSP3:
>+			reserved_bits = GENMASK_ULL(5, 0);
>+			break;
>+		case MSR_IA32_FRED_SSP1 ... MSR_IA32_FRED_SSP3:
>+			reserved_bits = GENMASK_ULL(2, 0);
>+			break;
>+		default:
>+			WARN_ON_ONCE(1);
>+			return 1;
>+		}
>+		if (data & reserved_bits)
>+			return 1;
>+		break;
> 	}
> 
> 	msr.data = data;
>@@ -1914,6 +1948,10 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
> 		    !guest_cpu_cap_has(vcpu, X86_FEATURE_RDPID))
> 			return 1;
> 		break;
>+	case MSR_IA32_FRED_RSP0 ... MSR_IA32_FRED_CONFIG:
>+		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
>+			return 1;
>+		break;
> 	}
> 
> 	msr.index = index;
>@@ -7365,6 +7403,10 @@ 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_FRED_RSP0 ... MSR_IA32_FRED_CONFIG:
>+		if (!kvm_cpu_cap_has(X86_FEATURE_FRED))
>+			return;
>+		break;
> 	default:
> 		break;
> 	}
>-- 
>2.50.1
>