diff --git a/lib/libsys/amd64/Makefile.sys b/lib/libsys/amd64/Makefile.sys
--- a/lib/libsys/amd64/Makefile.sys
+++ b/lib/libsys/amd64/Makefile.sys
@@ -3,6 +3,7 @@
 	amd64_get_gsbase.c \
 	amd64_set_fsbase.c \
 	amd64_set_gsbase.c \
+	amd64_set_tlsbase.c \
 	rfork_thread.S
 
 MDASM=	vfork.S cerror.S getcontext.S
diff --git a/lib/libsys/amd64/Symbol.sys.map b/lib/libsys/amd64/Symbol.sys.map
--- a/lib/libsys/amd64/Symbol.sys.map
+++ b/lib/libsys/amd64/Symbol.sys.map
@@ -13,6 +13,10 @@
 	 x86_pkru_unprotect_range;
 };
 
+FBSD_1.8 {
+	amd64_set_tlsbase;
+};
+
 FBSDprivate_1.0 {
 	_vfork;
 };
diff --git a/lib/libsys/amd64/amd64_set_tlsbase.c b/lib/libsys/amd64/amd64_set_tlsbase.c
new file mode 100644
--- /dev/null
+++ b/lib/libsys/amd64/amd64_set_tlsbase.c
@@ -0,0 +1,51 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2025 The FreeBSD Foundation
+ * All rights reserved.
+ *
+ * This software was developed by Konstantin Belousov
+ * under sponsorship from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#define _WANT_P_OSREL
+#include <sys/param.h>
+#include <machine/cpufunc.h>
+#include <machine/specialreg.h>
+#include <machine/sysarch.h>
+#include <x86/ifunc.h>
+#include "libc_private.h"
+
+static int
+amd64_set_tlsbase_syscall(void *addr)
+{
+	return (sysarch(AMD64_SET_TLSBASE, &addr));
+}
+
+DEFINE_UIFUNC(, int, amd64_set_tlsbase, (void *))
+{
+	if (__getosreldate() >= P_OSREL_TLSBASE)
+		return (amd64_set_tlsbase_syscall);
+	return (amd64_set_fsbase);
+}
diff --git a/libexec/rtld-elf/amd64/reloc.c b/libexec/rtld-elf/amd64/reloc.c
--- a/libexec/rtld-elf/amd64/reloc.c
+++ b/libexec/rtld-elf/amd64/reloc.c
@@ -543,10 +543,12 @@
 	addr = allocate_tls(objs, 0, TLS_TCB_SIZE, TLS_TCB_ALIGN);
 
 	/*
-	 * This does not use _tcb_set() as it calls amd64_set_fsbase()
+	 * This does not use _tcb_set() as it calls amd64_set_tlsbase()
 	 * which is an ifunc and rtld must not use ifuncs.
 	 */
-	if (__getosreldate() >= P_OSREL_WRFSBASE &&
+	if (__getosreldate() >= P_OSREL_TLSBASE)
+		sysarch(AMD64_SET_TLSBASE, &addr);
+	else if (__getosreldate() >= P_OSREL_WRFSBASE &&
 	    (cpu_stdext_feature & CPUID_STDEXT_FSGSBASE) != 0)
 		wrfsbase((uintptr_t)addr);
 	else
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/ptrace_machdep.c b/sys/amd64/amd64/ptrace_machdep.c
--- a/sys/amd64/amd64/ptrace_machdep.c
+++ b/sys/amd64/amd64/ptrace_machdep.c
@@ -368,6 +368,27 @@
 		cpu_ptrace_setbase(td, req, rv);
 		break;
 
+	case PT_GETTLSBASE:
+		pcb = td->td_pcb;
+		if ((pcb->pcb_flags & PCB_TLSBASE) != 0)
+			error = copyout(&pcb->pcb_tlsbase, addr, sizeof(*r));
+		else
+			error = ESRCH;
+		break;
+
+	case PT_SETTLSBASE:
+		pcb = td->td_pcb;
+		error = copyin(addr, &rv, sizeof(rv));
+		if (error != 0)
+			break;
+		if (rv >= td->td_proc->p_sysent->sv_maxuser) {
+			error = EINVAL;
+			break;
+		}
+		pcb->pcb_tlsbase = rv;
+		set_pcb_flags(pcb, PCB_TLSBASE);
+		break;
+
 	default:
 		error = EINVAL;
 		break;
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/sys/param.h b/sys/sys/param.h
--- a/sys/sys/param.h
+++ b/sys/sys/param.h
@@ -73,7 +73,7 @@
  * cannot include sys/param.h and should only be updated here.
  */
 #undef __FreeBSD_version
-#define __FreeBSD_version 1500043
+#define __FreeBSD_version 1500044
 
 /*
  * __FreeBSD_kernel__ indicates that this system uses the kernel of FreeBSD,
@@ -106,6 +106,7 @@
 #define	P_OSREL_POWERPC_NEW_AUX_ARGS	1300070
 #define	P_OSREL_TIDPID			1400079
 #define	P_OSREL_ARM64_SPSR		1400084
+#define	P_OSREL_TLSBASE			1500044
 
 #define	P_OSREL_MAJOR(x)		((x) / 100000)
 #endif
diff --git a/sys/x86/include/frame.h b/sys/x86/include/frame.h
--- a/sys/x86/include/frame.h
+++ b/sys/x86/include/frame.h
@@ -152,6 +152,7 @@
 #define	TF_HASSEGS	0x1
 #define	TF_HASBASES	0x2
 #define	TF_HASFPXSTATE	0x4
+#define	TF_RESERV0	0x8 /* no tlsbase in the trapframe */
 #endif /* __amd64__ */
 
 #endif /* _MACHINE_FRAME_H_ */
diff --git a/sys/x86/include/ptrace.h b/sys/x86/include/ptrace.h
--- a/sys/x86/include/ptrace.h
+++ b/sys/x86/include/ptrace.h
@@ -54,6 +54,8 @@
 #define	PT_SETFSBASE	(PT_FIRSTMACH + 8)
 #define	PT_GETGSBASE	(PT_FIRSTMACH + 9)
 #define	PT_SETGSBASE	(PT_FIRSTMACH + 10)
+#define	PT_GETTLSBASE	(PT_FIRSTMACH + 11)
+#define	PT_SETTLSBASE	(PT_FIRSTMACH + 12)
 
 /* Argument structure for PT_GETXSTATE_INFO. */
 struct ptrace_xstate_info {
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
diff --git a/sys/x86/include/tls.h b/sys/x86/include/tls.h
--- a/sys/x86/include/tls.h
+++ b/sys/x86/include/tls.h
@@ -69,7 +69,7 @@
 _tcb_set(struct tcb *tcb)
 {
 #ifdef __amd64__
-	amd64_set_fsbase(tcb);
+	amd64_set_tlsbase(tcb);
 #else
  	i386_set_gsbase(tcb);
 #endif
diff --git a/sys/x86/include/ucontext.h b/sys/x86/include/ucontext.h
--- a/sys/x86/include/ucontext.h
+++ b/sys/x86/include/ucontext.h
@@ -99,7 +99,9 @@
 #define	_MC_HASSEGS	0x1
 #define	_MC_HASBASES	0x2
 #define	_MC_HASFPXSTATE	0x4
-#define	_MC_FLAG_MASK	(_MC_HASSEGS | _MC_HASBASES | _MC_HASFPXSTATE)
+#define	_MC_HASTLSBASE	0x8
+#define	_MC_FLAG_MASK	(_MC_HASSEGS | _MC_HASBASES | _MC_HASFPXSTATE | \
+    _MC_HASTLSBASE)
 
 typedef struct __mcontext {
 	/*
@@ -158,7 +160,9 @@
 	__register_t	mc_xfpustate;
 	__register_t	mc_xfpustate_len;
 
-	long	mc_spare[4];
+	__register_t	mc_tlsbase;
+
+	long	mc_spare[3];
 } mcontext_t;
 #endif /* __amd64__ */