diff --git a/sys/amd64/amd64/exec_machdep.c b/sys/amd64/amd64/exec_machdep.c
--- a/sys/amd64/amd64/exec_machdep.c
+++ b/sys/amd64/amd64/exec_machdep.c
@@ -209,6 +209,8 @@
 	regs->tf_fs = _ufssel;
 	regs->tf_gs = _ugssel;
 	regs->tf_flags = TF_HASSEGS;
+	if ((pcb->pcb_flags & PCB_TLSBASE) != 0)
+		pcb->pcb_fsbase = pcb->pcb_tlsbase;
 	PROC_LOCK(p);
 	mtx_lock(&psp->ps_mtx);
 }
@@ -379,9 +381,9 @@
 		user_ldt_free(td);
 
 	update_pcb_bases(pcb);
-	pcb->pcb_fsbase = 0;
+	pcb->pcb_fsbase = pcb->pcb_tlsbase = 0;
 	pcb->pcb_gsbase = 0;
-	clear_pcb_flags(pcb, PCB_32BIT);
+	clear_pcb_flags(pcb, PCB_32BIT | PCB_TLSBASE);
 	pcb->pcb_initial_fpucw = __INITIAL_FPUCW__;
 
 	saved_rflags = regs->tf_rflags & PSL_T;
diff --git a/sys/amd64/amd64/sys_machdep.c b/sys/amd64/amd64/sys_machdep.c
--- a/sys/amd64/amd64/sys_machdep.c
+++ b/sys/amd64/amd64/sys_machdep.c
@@ -206,6 +206,8 @@
 	case AMD64_GET_XFPUSTATE:
 	case AMD64_SET_PKRU:
 	case AMD64_CLEAR_PKRU:
+	case AMD64_GET_TLSBASE:
+	case AMD64_SET_TLSBASE:
 		break;
 
 	case I386_SET_IOPERM:
@@ -311,14 +313,27 @@
 		error = copyout(&pcb->pcb_fsbase, uap->parms,
 		    sizeof(pcb->pcb_fsbase));
 		break;
+	case AMD64_GET_TLSBASE:
+		if ((pcb->pcb_flags & PCB_TLSBASE) == 0) {
+			error = ESRCH;
+		} else {
+			error = copyout(&pcb->pcb_tlsbase, uap->parms,
+			    sizeof(pcb->pcb_tlsbase));
+		}
+		break;
 
 	case AMD64_SET_FSBASE:
+	case AMD64_SET_TLSBASE:
 		error = copyin(uap->parms, &a64base, sizeof(a64base));
 		if (error == 0) {
 			if (a64base < curproc->p_sysent->sv_maxuser) {
 				set_pcb_flags(pcb, PCB_FULL_IRET);
 				pcb->pcb_fsbase = a64base;
 				td->td_frame->tf_fs = _ufssel;
+				if (uap->op == AMD64_SET_TLSBASE) {
+					pcb->pcb_tlsbase = a64base;
+					set_pcb_flags(pcb, PCB_TLSBASE);
+				}
 			} else
 				error = EINVAL;
 		}
diff --git a/sys/amd64/amd64/vm_machdep.c b/sys/amd64/amd64/vm_machdep.c
--- a/sys/amd64/amd64/vm_machdep.c
+++ b/sys/amd64/amd64/vm_machdep.c
@@ -156,7 +156,7 @@
 
 	/* Kernel threads start with clean FPU and segment bases. */
 	if ((td2->td_pflags & TDP_KTHREAD) != 0) {
-		pcb2->pcb_fsbase = 0;
+		pcb2->pcb_fsbase = pcb2->pcb_tlsbase = 0;
 		pcb2->pcb_gsbase = 0;
 		clear_pcb_flags(pcb2, PCB_FPUINITDONE | PCB_USERFPUINITDONE |
 		    PCB_KERNFPU | PCB_KERNFPU_THR);
@@ -182,7 +182,7 @@
 	 * pcb2->pcb_savefpu:	cloned above.
 	 * pcb2->pcb_flags:	cloned above.
 	 * pcb2->pcb_onfault:	cloned above (always NULL here?).
-	 * pcb2->pcb_[fg]sbase:	cloned above
+	 * pcb2->pcb_[f,g,tls]sbase:	cloned above
 	 */
 
 	pcb2->pcb_tssp = NULL;
@@ -663,14 +663,14 @@
 		return (EINVAL);
 
 	pcb = td->td_pcb;
-	set_pcb_flags(pcb, PCB_FULL_IRET);
+	set_pcb_flags(pcb, PCB_FULL_IRET | PCB_TLSBASE);
 #ifdef COMPAT_FREEBSD32
 	if (SV_PROC_FLAG(td->td_proc, SV_ILP32)) {
 		pcb->pcb_gsbase = (register_t)tls_base;
 		return (0);
 	}
 #endif
-	pcb->pcb_fsbase = (register_t)tls_base;
+	pcb->pcb_fsbase = pcb->pcb_tlsbase = (register_t)tls_base;
 	return (0);
 }
 
diff --git a/sys/amd64/ia32/ia32_signal.c b/sys/amd64/ia32/ia32_signal.c
--- a/sys/amd64/ia32/ia32_signal.c
+++ b/sys/amd64/ia32/ia32_signal.c
@@ -958,4 +958,5 @@
 
 	/* Return via doreti so that we can change to a different %cs */
 	set_pcb_flags(pcb, PCB_32BIT | PCB_FULL_IRET);
+	clear_pcb_flags(pcb, PCB_TLSBASE);
 }
diff --git a/sys/amd64/include/pcb.h b/sys/amd64/include/pcb.h
--- a/sys/amd64/include/pcb.h
+++ b/sys/amd64/include/pcb.h
@@ -84,6 +84,7 @@
 #define	PCB_KERNFPU_THR	0x0020	/* fpu_kern_thread() */
 #define	PCB_32BIT	0x0040	/* process has 32 bit context (segs etc) */
 #define	PCB_FPUNOSAVE	0x0080	/* no save area for current FPU ctx */
+#define	PCB_TLSBASE	0x0100	/* tlsbase was set */
 
 	uint16_t	pcb_initial_fpucw;
 
@@ -104,7 +105,8 @@
 
 	struct savefpu	*pcb_save;
 
-	uint64_t	pcb_pad[5];
+	register_t	pcb_tlsbase;	/* not same as pcb_fsbase */
+	uint64_t	pcb_pad[4];
 };
 
 /* Per-CPU state saved during suspend and resume. */
diff --git a/sys/amd64/linux/linux_sysvec.c b/sys/amd64/linux/linux_sysvec.c
--- a/sys/amd64/linux/linux_sysvec.c
+++ b/sys/amd64/linux/linux_sysvec.c
@@ -247,7 +247,7 @@
 
 	pcb->pcb_fsbase = 0;
 	pcb->pcb_gsbase = 0;
-	clear_pcb_flags(pcb, PCB_32BIT);
+	clear_pcb_flags(pcb, PCB_32BIT | PCB_TLSBASE);
 	pcb->pcb_initial_fpucw = __LINUX_NPXCW__;
 	set_pcb_flags(pcb, PCB_FULL_IRET);
 
diff --git a/sys/amd64/linux32/linux32_sysvec.c b/sys/amd64/linux32/linux32_sysvec.c
--- a/sys/amd64/linux32/linux32_sysvec.c
+++ b/sys/amd64/linux32/linux32_sysvec.c
@@ -580,6 +580,10 @@
 	if (td->td_proc->p_md.md_ldt != NULL)
 		user_ldt_free(td);
 
+	/* Do full restore on return so that we can change to a different %cs */
+	set_pcb_flags(pcb, PCB_32BIT | PCB_FULL_IRET);
+	clear_pcb_flags(pcb, PCB_TLSBASE);
+
 	critical_enter();
 	wrmsr(MSR_FSBASE, 0);
 	wrmsr(MSR_KGSBASE, 0);	/* User value while we're in the kernel */
@@ -605,9 +609,6 @@
 	x86_clear_dbregs(pcb);
 
 	fpstate_drop(td);
-
-	/* Do full restore on return so that we can change to a different %cs */
-	set_pcb_flags(pcb, PCB_32BIT | PCB_FULL_IRET);
 }
 
 /*
diff --git a/sys/x86/include/sysarch.h b/sys/x86/include/sysarch.h
--- a/sys/x86/include/sysarch.h
+++ b/sys/x86/include/sysarch.h
@@ -61,6 +61,8 @@
 #define	AMD64_GET_XFPUSTATE	132
 #define	AMD64_SET_PKRU		133
 #define	AMD64_CLEAR_PKRU	134
+#define	AMD64_GET_TLSBASE	135
+#define	AMD64_SET_TLSBASE	136
 
 /* Flags for AMD64_SET_PKRU */
 #define	AMD64_PKRU_EXCL		0x0001