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* [PATCH v2] KVM: SVM: Add Bus Lock Detect support
@ 2025-11-21  8:12 Shivansh Dhiman
  2026-04-24  4:45 ` Shivansh Dhiman
  0 siblings, 1 reply; 2+ messages in thread
From: Shivansh Dhiman @ 2025-11-21  8:12 UTC (permalink / raw)
  To: kvm, x86, seanjc, pbonzini
  Cc: yosry.ahmed, thomas.lendacky, santosh.shukla, nikunj.dadhania,
	ravi.bangoria, shivansh.dhiman

From: Ravi Bangoria <ravi.bangoria@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.

While adding Bus Lock Detect support, also fix DR6 handling in nested
virtualization. Using DR6_FIXED_1 to prevent reset of BLD bit (bit 11)
between VMRUNs. However, it preserves DR6_RTM, which is a reserved bit
on AMD processors. So, DR6_RTM bit must always be set to 1.

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

 v1: https://lore.kernel.org/all/20240808062937.1149-5-ravi.bangoria@amd.com
---
 arch/x86/kvm/svm/nested.c |  3 ++-
 arch/x86/kvm/svm/svm.c    | 17 ++++++++++++++++-
 arch/x86/kvm/svm/svm.h    |  2 +-
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index da6e80b3ac35..5af7fbf5c536 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -665,7 +665,8 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
 	/* These bits will be set properly on the first execution when new_vmc12 is true */
 	if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_DR))) {
 		vmcb02->save.dr7 = svm->nested.save.dr7 | DR7_FIXED_1;
-		svm->vcpu.arch.dr6  = svm->nested.save.dr6 | DR6_ACTIVE_LOW;
+		/* DR6_RTM is a reserved bit on AMD and as such must be set to 1 */
+		svm->vcpu.arch.dr6  = svm->nested.save.dr6 | DR6_FIXED_1 | DR6_RTM;
 		vmcb_mark_dirty(vmcb02, VMCB_DR);
 	}
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 10c21e4c5406..ceef5b697e11 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -831,6 +831,9 @@ void svm_update_lbrv(struct kvm_vcpu *vcpu)
 			    (is_guest_mode(vcpu) && guest_cpu_cap_has(vcpu, X86_FEATURE_LBRV) &&
 			    (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK));
 
+	/* Bus Lock Detect depends on LBR Virtualization */
+	enable_lbrv |= (svm->vmcb->save.dbgctl & DEBUGCTLMSR_BUS_LOCK_DETECT);
+
 	if (enable_lbrv && !current_enable_lbrv)
 		__svm_enable_lbrv(vcpu);
 	else if (!enable_lbrv && current_enable_lbrv)
@@ -2984,6 +2987,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 			data &= ~DEBUGCTLMSR_BTF;
 		}
 
+		if ((data & DEBUGCTLMSR_BUS_LOCK_DETECT) &&
+		    !guest_cpu_cap_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
+			return 1;
+
 		if (data & DEBUGCTL_RESERVED_BITS)
 			return 1;
 
@@ -5258,8 +5265,16 @@ 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);
 }
 
 static __init int svm_hardware_setup(void)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index c856d8e0f95e..8e8d31788c9c 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -675,7 +675,7 @@ BUILD_SVM_MSR_BITMAP_HELPERS(bool, test, test)
 BUILD_SVM_MSR_BITMAP_HELPERS(void, clear, __clear)
 BUILD_SVM_MSR_BITMAP_HELPERS(void, set, __set)
 
-#define DEBUGCTL_RESERVED_BITS (~DEBUGCTLMSR_LBR)
+#define DEBUGCTL_RESERVED_BITS (~(DEBUGCTLMSR_LBR | DEBUGCTLMSR_BUS_LOCK_DETECT))
 
 /* svm.c */
 extern bool dump_invalid_vmcb;
-- 
2.43.0


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