[PATCH 6.18.y 5/6] KVM: x86: Virtualize AMD CPUID faulting

[Anil Altinay <aaltinay@google.com>]

From: Jim Mattson <jmattson@google.com>

commit e93a93f11490992a477546d38d3a37b9b8dc6a5f upstream.

On AMD CPUs, CPUID faulting support is advertised via
CPUID.80000021H:EAX.CpuidUserDis[bit 17] and enabled by setting
HWCR.CpuidUserDis[bit 35].

Advertise the feature to userspace regardless of host CPU support. Allow
writes to HWCR to set bit 35 when the guest CPUID advertises
CpuidUserDis. Update cpuid_fault_enabled() to check HWCR.CpuidUserDis as
well as MSR_FEATURE_ENABLES.CPUID_GP_ON_CPL_GT_0.

Unlike VMX, SVM prioritizes the CPUID intercept over the #GP induced by
CPUID faulting.[1] This behavior has been confirmed on a Turin CPU (F/M/S
1AH/2/1).

Link: https://lore.kernel.org/r/DS7PR12MB82011943131DF5415365E19E940B2@DS7PR12MB8201.namprd12.prod.outlook.com [1]
Signed-off-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20260527174347.2356165-5-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Anil Altinay <aaltinay@google.com>
---
 arch/x86/include/asm/msr-index.h |  1 +
 arch/x86/kvm/cpuid.c             |  2 +-
 arch/x86/kvm/cpuid.h             |  5 +++--
 arch/x86/kvm/x86.c               | 18 ++++++++++++------
 4 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index d0a0cc8e8bd9..71bbdd920ac3 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -863,6 +863,7 @@
 #define MSR_K7_HWCR_IRPERF_EN_BIT	30
 #define MSR_K7_HWCR_IRPERF_EN		BIT_ULL(MSR_K7_HWCR_IRPERF_EN_BIT)
 #define MSR_K7_HWCR_CPUID_USER_DIS_BIT	35
+#define MSR_K7_HWCR_CPUID_USER_DIS	BIT_ULL(MSR_K7_HWCR_CPUID_USER_DIS_BIT)
 #define MSR_K7_FID_VID_CTL		0xc0010041
 #define MSR_K7_FID_VID_STATUS		0xc0010042
 #define MSR_K7_HWCR_CPB_DIS_BIT		25
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 3f2689fdc304..5edd00ac7fa0 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1220,7 +1220,7 @@ void kvm_set_cpu_caps(void)
 		F(PREFETCHI),
 		EMULATED_F(NO_SMM_CTL_MSR),
 		/* PrefetchCtlMsr */
-		/* GpOnUserCpuid */
+		EMULATED_F(GP_ON_USER_CPUID),
 		/* EPSF */
 		SYNTHESIZED_F(SBPB),
 		SYNTHESIZED_F(IBPB_BRTYPE),
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index 872bc99c55ed..0d757852a1e5 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -177,8 +177,9 @@ static inline int guest_cpuid_stepping(struct kvm_vcpu *vcpu)
 
 static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)
 {
-	return vcpu->arch.msr_misc_features_enables &
-		  MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
+	return (vcpu->arch.msr_misc_features_enables &
+		MSR_MISC_FEATURES_ENABLES_CPUID_FAULT) ||
+		(vcpu->arch.msr_hwcr & MSR_K7_HWCR_CPUID_USER_DIS);
 }
 
 static inline bool kvm_is_cpuid_allowed(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a91599e5d8db..8e2ef1e45bc7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3656,22 +3656,28 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_EFER:
 		return set_efer(vcpu, msr_info);
-	case MSR_K7_HWCR:
-		data &= ~(u64)0x40;	/* ignore flush filter disable */
-		data &= ~(u64)0x100;	/* ignore ignne emulation enable */
-		data &= ~(u64)0x8;	/* ignore TLB cache disable */
-
+	case MSR_K7_HWCR: {
 		/*
 		 * Allow McStatusWrEn and TscFreqSel. (Linux guests from v3.2
 		 * through at least v6.6 whine if TscFreqSel is clear,
 		 * depending on F/M/S.
 		 */
-		if (data & ~(BIT_ULL(18) | BIT_ULL(24))) {
+		u64 valid = BIT_ULL(18) | BIT_ULL(24);
+
+		data &= ~(u64)0x40;	/* ignore flush filter disable */
+		data &= ~(u64)0x100;	/* ignore ignne emulation enable */
+		data &= ~(u64)0x8;	/* ignore TLB cache disable */
+
+		if (guest_cpu_cap_has(vcpu, X86_FEATURE_GP_ON_USER_CPUID))
+			valid |= MSR_K7_HWCR_CPUID_USER_DIS;
+
+		if (data & ~valid) {
 			kvm_pr_unimpl_wrmsr(vcpu, msr, data);
 			return 1;
 		}
 		vcpu->arch.msr_hwcr = data;
 		break;
+	}
 	case MSR_FAM10H_MMIO_CONF_BASE:
 		if (data != 0) {
 			kvm_pr_unimpl_wrmsr(vcpu, msr, data);
-- 
2.56.0.385.gd3acb90ef8-goog