Index: sys/arch/amd64/amd64/genassym.cf
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/amd64/genassym.cf,v
retrieving revision 1.68
diff -u -r1.68 genassym.cf
--- sys/arch/amd64/amd64/genassym.cf	30 Mar 2018 10:01:36 -0000	1.68
+++ sys/arch/amd64/amd64/genassym.cf	14 Jul 2018 16:04:39 -0000
@@ -190,6 +190,7 @@
 define	PCB_COMPAT32		PCB_COMPAT32
 define	PCB_FS			offsetof(struct pcb, pcb_fs)
 define	PCB_GS			offsetof(struct pcb, pcb_gs)
+define	PCB_DBREGS		offsetof(struct pcb, pcb_dbregs)
 
 define	TF_RDI			offsetof(struct trapframe, tf_rdi)
 define	TF_RSI			offsetof(struct trapframe, tf_rsi)
Index: sys/arch/amd64/amd64/locore.S
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/amd64/locore.S,v
retrieving revision 1.171
diff -u -r1.171 locore.S
--- sys/arch/amd64/amd64/locore.S	14 Jul 2018 14:29:40 -0000	1.171
+++ sys/arch/amd64/amd64/locore.S	14 Jul 2018 16:04:39 -0000
@@ -1142,6 +1142,40 @@
 #endif
 .Lring0_switched:
 
+	/* New lwp using dbregs? If so, restore. */
+	cmpq	$0,PCB_DBREGS(%r14)
+	jne	.Ldbregs_restore
+
+	/* Old lwp using dbregs? If so, reset. */
+	testq	%r13,%r13	/* oldlwp = NULL ? */
+	jz	.Ldbregs_done
+	movq	L_PCB(%r13),%rax
+	cmpq	$0,PCB_DBREGS(%rax)
+	je	.Ldbregs_done
+
+	/* Reset the dbregs. */
+	xorq	%rax,%rax
+	movq	%rax,%dr7
+	movq	%rax,%dr6
+	jmp	.Ldbregs_done
+
+	/* Restore the dbregs. */
+.Ldbregs_restore:
+	movq	PCB_DBREGS(%r14),%rax
+	movq	(0*8)(%rax),%rdx
+	movq	%rdx,%dr0
+	movq	(1*8)(%rax),%rdx
+	movq	%rdx,%dr1
+	movq	(2*8)(%rax),%rdx
+	movq	%rdx,%dr2
+	movq	(3*8)(%rax),%rdx
+	movq	%rdx,%dr3
+	movq	(6*8)(%rax),%rdx
+	movq	%rdx,%dr6
+	movq	(7*8)(%rax),%rdx
+	movq	%rdx,%dr7
+.Ldbregs_done:
+
 #ifndef XEN
 	movb	_C_LABEL(x86_fpu_eager),%dl
 	testb	%dl,%dl
Index: sys/arch/amd64/amd64/machdep.c
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/amd64/machdep.c,v
retrieving revision 1.306
diff -u -r1.306 machdep.c
--- sys/arch/amd64/amd64/machdep.c	12 Jul 2018 19:48:16 -0000	1.306
+++ sys/arch/amd64/amd64/machdep.c	14 Jul 2018 16:04:39 -0000
@@ -1368,6 +1368,7 @@
 	if (pcb->pcb_dbregs != NULL) {
 		pool_put(&x86_dbregspl, pcb->pcb_dbregs);
 		pcb->pcb_dbregs = NULL;
+		x86_dbregs_clear(l);
 	}
 
 	l->l_proc->p_flag &= ~PK_32;
Index: sys/arch/amd64/amd64/netbsd32_machdep.c
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/amd64/netbsd32_machdep.c,v
retrieving revision 1.115
diff -u -r1.115 netbsd32_machdep.c
--- sys/arch/amd64/amd64/netbsd32_machdep.c	7 Dec 2017 23:11:50 -0000	1.115
+++ sys/arch/amd64/amd64/netbsd32_machdep.c	14 Jul 2018 16:04:39 -0000
@@ -62,6 +62,7 @@
 #include <sys/ptrace.h>
 #include <sys/kauth.h>
 
+#include <x86/dbregs.h>
 #include <x86/fpu.h>
 #include <machine/frame.h>
 #include <machine/reg.h>
@@ -141,6 +142,7 @@
 	if (pcb->pcb_dbregs != NULL) {
 		pool_put(&x86_dbregspl, pcb->pcb_dbregs);
 		pcb->pcb_dbregs = NULL;
+		x86_dbregs_clear(l);
 	}
 
 	p->p_flag |= PK_32;
Index: sys/arch/amd64/amd64/trap.c
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/amd64/trap.c,v
retrieving revision 1.113
diff -u -r1.113 trap.c
--- sys/arch/amd64/amd64/trap.c	25 Feb 2018 13:09:33 -0000	1.113
+++ sys/arch/amd64/amd64/trap.c	14 Jul 2018 16:04:39 -0000
@@ -93,6 +93,7 @@
 
 #include <machine/cpufunc.h>
 #include <x86/fpu.h>
+#include <x86/dbregs.h>
 #include <machine/psl.h>
 #include <machine/reg.h>
 #include <machine/trap.h>
Index: sys/arch/amd64/include/userret.h
===================================================================
RCS file: /cvsroot/src/sys/arch/amd64/include/userret.h,v
retrieving revision 1.12
diff -u -r1.12 userret.h
--- sys/arch/amd64/include/userret.h	23 Feb 2017 03:34:22 -0000	1.12
+++ sys/arch/amd64/include/userret.h	14 Jul 2018 16:04:39 -0000
@@ -67,7 +67,6 @@
  */
 
 #include <sys/userret.h>
-#include <x86/dbregs.h>
 
 static __inline void userret(struct lwp *);
 
@@ -78,13 +77,6 @@
 static __inline void
 userret(struct lwp *l)
 {
-	struct pcb *pcb = lwp_getpcb(l);
-
 	/* Invoke MI userret code */
 	mi_userret(l);
-
-	if (pcb->pcb_dbregs)
-		x86_dbregs_set(l);
-	else
-		x86_dbregs_clear(l);
 }
Index: sys/arch/x86/x86/vm_machdep.c
===================================================================
RCS file: /cvsroot/src/sys/arch/x86/x86/vm_machdep.c,v
retrieving revision 1.35
diff -u -r1.35 vm_machdep.c
--- sys/arch/x86/x86/vm_machdep.c	1 Jul 2018 08:32:41 -0000	1.35
+++ sys/arch/x86/x86/vm_machdep.c	14 Jul 2018 16:04:40 -0000
@@ -291,6 +291,7 @@
 	if (pcb->pcb_dbregs) {
 		pool_put(&x86_dbregspl, pcb->pcb_dbregs);
 		pcb->pcb_dbregs = NULL;
+		x86_dbregs_clear(l);
 	}
 }