KVM: x86: DR7.GD should be cleared upon any #DB exception

Intel SDM 17.2.4 (Debug Control Register (DR7)) says: "The processor clears the
GD flag upon entering to the debug exception handler." This sentence may be
misunderstood as if it happens only on #DB due to debug-register protection,
but it happens regardless to the cause of the #DB.

Fix the behavior to match both real hardware and Bochs.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -5185,9 +5185,7 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 			vcpu->run->exit_reason = KVM_EXIT_DEBUG;
 			return 0;
 		} else {
-			vcpu->arch.dr7 &= ~DR7_GD;
 			vcpu->arch.dr6 |= DR6_BD | DR6_RTM;
-			vmcs_writel(GUEST_DR7, vcpu->arch.dr7);
 			kvm_queue_exception(vcpu, DB_VECTOR);
 			return 1;
 		}

arch/x86/kvm/x86.c

@@ -5965,6 +5965,12 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
 			__kvm_set_rflags(vcpu, kvm_get_rflags(vcpu) |
 					     X86_EFLAGS_RF);
 
+		if (vcpu->arch.exception.nr == DB_VECTOR &&
+		    (vcpu->arch.dr7 & DR7_GD)) {
+			vcpu->arch.dr7 &= ~DR7_GD;
+			kvm_update_dr7(vcpu);
+		}
+
 		kvm_x86_ops->queue_exception(vcpu, vcpu->arch.exception.nr,
 					  vcpu->arch.exception.has_error_code,
 					  vcpu->arch.exception.error_code,