KVM: X86: Rename DR6_INIT to DR6_ACTIVE_LOW

DR6_INIT contains the 1-reserved bits as well as the bit that is cleared
to 0 when the condition (e.g. RTM) happens. The value can be used to
initialize dr6 and also be the XOR mask between the #DB exit
qualification (or payload) and DR6.

Concerning that DR6_INIT is used as initial value only once, rename it
to DR6_ACTIVE_LOW and apply it in other places, which would make the
incoming changes for bus lock debug exception more simple.
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20210202090433.13441-2-chenyi.qiang@intel.com>
[Define DR6_FIXED_1 from DR6_ACTIVE_LOW and DR6_VOLATILE. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -203,9 +203,17 @@ enum x86_intercept_stage;
 #define DR6_BS		(1 << 14)
 #define DR6_BT		(1 << 15)
 #define DR6_RTM		(1 << 16)
-#define DR6_FIXED_1	0xfffe0ff0
-#define DR6_INIT	0xffff0ff0
+/*
+ * DR6_ACTIVE_LOW combines fixed-1 and active-low bits.
+ * We can regard all the bits in DR6_FIXED_1 as active_low bits;
+ * they will never be 0 for now, but when they are defined
+ * in the future it will require no code change.
+ *
+ * DR6_ACTIVE_LOW is also used as the init/reset value for DR6.
+ */
+#define DR6_ACTIVE_LOW	0xffff0ff0
 #define DR6_VOLATILE	0x0001e00f
+#define DR6_FIXED_1	(DR6_ACTIVE_LOW & ~DR6_VOLATILE)
 
 #define DR7_BP_EN_MASK	0x000000ff
 #define DR7_GE		(1 << 9)

arch/x86/kvm/emulate.c

@@ -4329,7 +4329,7 @@ static int check_dr_read(struct x86_emulate_ctxt *ctxt)
 
 		ctxt->ops->get_dr(ctxt, 6, &dr6);
 		dr6 &= ~DR_TRAP_BITS;
-		dr6 |= DR6_BD | DR6_RTM;
+		dr6 |= DR6_BD | DR6_ACTIVE_LOW;
 		ctxt->ops->set_dr(ctxt, 6, dr6);
 		return emulate_db(ctxt);
 	}

arch/x86/kvm/svm/nested.c

@@ -392,7 +392,7 @@ static void nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *vmcb12)
 	svm->vmcb->save.rsp = vmcb12->save.rsp;
 	svm->vmcb->save.rip = vmcb12->save.rip;
 	svm->vmcb->save.dr7 = vmcb12->save.dr7 | DR7_FIXED_1;
-	svm->vcpu.arch.dr6  = vmcb12->save.dr6 | DR6_FIXED_1 | DR6_RTM;
+	svm->vcpu.arch.dr6  = vmcb12->save.dr6 | DR6_ACTIVE_LOW;
 	svm->vmcb->save.cpl = vmcb12->save.cpl;
 }
 

arch/x86/kvm/svm/svm.c

@@ -1865,7 +1865,7 @@ static void svm_sync_dirty_debug_regs(struct kvm_vcpu *vcpu)
 	get_debugreg(vcpu->arch.db[2], 2);
 	get_debugreg(vcpu->arch.db[3], 3);
 	/*
-	 * We cannot reset svm->vmcb->save.dr6 to DR6_FIXED_1|DR6_RTM here,
+	 * We cannot reset svm->vmcb->save.dr6 to DR6_ACTIVE_LOW here,
 	 * because db_interception might need it.  We can do it before vmentry.
 	 */
 	vcpu->arch.dr6 = svm->vmcb->save.dr6;
@@ -1916,7 +1916,7 @@ static int db_interception(struct vcpu_svm *svm)
 	if (!(svm->vcpu.guest_debug &
 	      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) &&
 		!svm->nmi_singlestep) {
-		u32 payload = (svm->vmcb->save.dr6 ^ DR6_RTM) & ~DR6_FIXED_1;
+		u32 payload = svm->vmcb->save.dr6 ^ DR6_ACTIVE_LOW;
 		kvm_queue_exception_p(&svm->vcpu, DB_VECTOR, payload);
 		return 1;
 	}
@@ -3783,7 +3783,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
 	if (unlikely(svm->vcpu.arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
 		svm_set_dr6(svm, vcpu->arch.dr6);
 	else
-		svm_set_dr6(svm, DR6_FIXED_1 | DR6_RTM);
+		svm_set_dr6(svm, DR6_ACTIVE_LOW);
 
 	clgi();
 	kvm_load_guest_xsave_state(vcpu);

arch/x86/kvm/vmx/nested.c

@@ -411,8 +411,8 @@ static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned long *exit
 		if (nr == DB_VECTOR) {
 			if (!has_payload) {
 				payload = vcpu->arch.dr6;
-				payload &= ~(DR6_FIXED_1 | DR6_BT);
-				payload ^= DR6_RTM;
+				payload &= ~DR6_BT;
+				payload ^= DR6_ACTIVE_LOW;
 			}
 			*exit_qual = payload;
 		} else

arch/x86/kvm/vmx/vmx.c

@@ -4876,7 +4876,7 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 			kvm_queue_exception_p(vcpu, DB_VECTOR, dr6);
 			return 1;
 		}
-		kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1 | DR6_RTM;
+		kvm_run->debug.arch.dr6 = dr6 | DR6_ACTIVE_LOW;
 		kvm_run->debug.arch.dr7 = vmcs_readl(GUEST_DR7);
 		fallthrough;
 	case BP_VECTOR:
@@ -5120,7 +5120,7 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 		 * guest debugging itself.
 		 */
 		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) {
-			vcpu->run->debug.arch.dr6 = DR6_BD | DR6_RTM | DR6_FIXED_1;
+			vcpu->run->debug.arch.dr6 = DR6_BD | DR6_ACTIVE_LOW;
 			vcpu->run->debug.arch.dr7 = dr7;
 			vcpu->run->debug.arch.pc = kvm_get_linear_rip(vcpu);
 			vcpu->run->debug.arch.exception = DB_VECTOR;

arch/x86/kvm/x86.c

@@ -486,19 +486,24 @@ void kvm_deliver_exception_payload(struct kvm_vcpu *vcpu)
 		 */
 		vcpu->arch.dr6 &= ~DR_TRAP_BITS;
 		/*
-		 * DR6.RTM is set by all #DB exceptions that don't clear it.
+		 * In order to reflect the #DB exception payload in guest
+		 * dr6, three components need to be considered: active low
+		 * bit, FIXED_1 bits and active high bits (e.g. DR6_BD,
+		 * DR6_BS and DR6_BT)
+		 * DR6_ACTIVE_LOW contains the FIXED_1 and active low bits.
+		 * In the target guest dr6:
+		 * FIXED_1 bits should always be set.
+		 * Active low bits should be cleared if 1-setting in payload.
+		 * Active high bits should be set if 1-setting in payload.
+		 *
+		 * Note, the payload is compatible with the pending debug
+		 * exceptions/exit qualification under VMX, that active_low bits
+		 * are active high in payload.
+		 * So they need to be flipped for DR6.
 		 */
-		vcpu->arch.dr6 |= DR6_RTM;
+		vcpu->arch.dr6 |= DR6_ACTIVE_LOW;
 		vcpu->arch.dr6 |= payload;
-		/*
-		 * Bit 16 should be set in the payload whenever the #DB
-		 * exception should clear DR6.RTM. This makes the payload
-		 * compatible with the pending debug exceptions under VMX.
-		 * Though not currently documented in the SDM, this also
-		 * makes the payload compatible with the exit qualification
-		 * for #DB exceptions under VMX.
-		 */
-		vcpu->arch.dr6 ^= payload & DR6_RTM;
+		vcpu->arch.dr6 ^= payload & DR6_ACTIVE_LOW;
 
 		/*
 		 * The #DB payload is defined as compatible with the 'pending
@@ -7211,7 +7216,7 @@ static int kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu)
 	struct kvm_run *kvm_run = vcpu->run;
 
 	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
-		kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM;
+		kvm_run->debug.arch.dr6 = DR6_BS | DR6_ACTIVE_LOW;
 		kvm_run->debug.arch.pc = kvm_get_linear_rip(vcpu);
 		kvm_run->debug.arch.exception = DB_VECTOR;
 		kvm_run->exit_reason = KVM_EXIT_DEBUG;
@@ -7255,7 +7260,7 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
 					   vcpu->arch.eff_db);
 
 		if (dr6 != 0) {
-			kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1 | DR6_RTM;
+			kvm_run->debug.arch.dr6 = dr6 | DR6_ACTIVE_LOW;
 			kvm_run->debug.arch.pc = eip;
 			kvm_run->debug.arch.exception = DB_VECTOR;
 			kvm_run->exit_reason = KVM_EXIT_DEBUG;
@@ -10118,7 +10123,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 
 	memset(vcpu->arch.db, 0, sizeof(vcpu->arch.db));
 	kvm_update_dr0123(vcpu);
-	vcpu->arch.dr6 = DR6_INIT;
+	vcpu->arch.dr6 = DR6_ACTIVE_LOW;
 	vcpu->arch.dr7 = DR7_FIXED_1;
 	kvm_update_dr7(vcpu);