[PATCH v4 5/5] KVM: SVM: Add Bus Lock Detect support

[Shivansh Dhiman <shivansh.dhiman@amd.com>]

Add Bus Lock Detect support in AMD SVM. Bus Lock Detect is enabled through
MSR_IA32_DEBUGCTLMSR and MSR_IA32_DEBUGCTLMSR is virtualized only if LBR
Virtualization is enabled. Add this dependency in the SVM.

Also, allow DEBUGCTLMSR_BUS_LOCK_DETECT only when the guest supports Bus
Lock Detect.

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Co-developed-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
Signed-off-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
---
Changelog:
v3 -> v4:
 * Fold in the DEBUGCTLMSR_BUS_LOCK_DETECT gating moved from patch 4.

v2 -> v3:
 * Split refactor and prep changes out into patches 1-4; this patch now
   only wires up the LBRV dependency and exposes the capability.

v2 Resend:
 * Rebased on top of tag: kvm-x86-next-2026.06.24.

v1 -> v2:
 * Rebased and used guest_cpu_cap_has() instead of guest_cpuid_has().

 v2 Resend: https://lore.kernel.org/kvm/20260629081018.60618-1-shivansh.dhiman@amd.com/
 v2: https://lore.kernel.org/kvm/20251121081228.426974-1-shivansh.dhiman@amd.com/
 v1: https://lore.kernel.org/all/20240808062937.1149-5-ravi.bangoria@amd.com
---
 arch/x86/kvm/svm/svm.c | 13 +++++++++++--
 arch/x86/kvm/svm/svm.h |  9 +++++++--
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index ccf85ea79d37..b0f217721873 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -882,7 +882,8 @@ void svm_update_lbrv(struct kvm_vcpu *vcpu)
 	bool current_enable_lbrv = svm->vmcb->control.misc_ctl2 & SVM_MISC2_ENABLE_V_LBR;
 	bool enable_lbrv = false;
 
-	if (svm->vmcb->save.dbgctl & DEBUGCTLMSR_LBR)
+	/* Bus Lock Detect in guest depends on LBR Virtualization */
+	if (svm->vmcb->save.dbgctl & (DEBUGCTLMSR_LBR | DEBUGCTLMSR_BUS_LOCK_DETECT))
 		enable_lbrv = true;
 
 	if (is_guest_mode(vcpu) && nested_lbrv_enabled(vcpu))
@@ -5594,9 +5595,17 @@ static __init void svm_set_cpu_caps(void)
 	 * Clear capabilities that are automatically configured by common code,
 	 * but that require explicit SVM support (that isn't yet implemented).
 	 */
-	kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
 	kvm_cpu_cap_clear(X86_FEATURE_MSR_IMM);
 
+	/*
+	 * LBR Virtualization must be enabled to support BusLockTrap inside the
+	 * guest, since BusLockTrap is enabled through MSR_IA32_DEBUGCTLMSR and
+	 * MSR_IA32_DEBUGCTLMSR is virtualized only if LBR Virtualization is
+	 * enabled.
+	 */
+	if (!lbrv)
+		kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
+
 	kvm_setup_xss_caps();
 	kvm_finalize_cpu_caps();
 }
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 696f1b4b8f8f..1d1ff6ed1049 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -783,9 +783,14 @@ BUILD_SVM_MSR_BITMAP_HELPERS(bool, test, test)
 BUILD_SVM_MSR_BITMAP_HELPERS(void, clear, __clear)
 BUILD_SVM_MSR_BITMAP_HELPERS(void, set, __set)
 
-static inline u64 svm_get_debugctl_reserved_bits(struct kvm_vcpu *vcpu __maybe_unused)
+static inline u64 svm_get_debugctl_reserved_bits(struct kvm_vcpu *vcpu)
 {
-	return ~DEBUGCTLMSR_LBR;
+	u64 debugctl = DEBUGCTLMSR_LBR;
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
+		debugctl |= DEBUGCTLMSR_BUS_LOCK_DETECT;
+
+	return ~debugctl;
 }
 
 /* svm.c */
-- 
2.43.0