Index: amd64/include/pcb.h
===================================================================
--- amd64/include/pcb.h	(revision 195227)
+++ amd64/include/pcb.h	(working copy)
@@ -56,6 +56,7 @@
 	register_t	pcb_fsbase;
 	register_t	pcb_gsbase;
 	u_long		pcb_flags;
+#define	PCB_SEGLOADED	0x01	/* Segment registers are loaded and valid. */
 #define	PCB_DBREGS	0x02	/* process using debug registers */
 #define	PCB_FPUINITDONE	0x08	/* fpu state is initialized */
 #define	PCB_GS32BIT	0x20	/* linux gs switch */
Index: amd64/amd64/vm_machdep.c
===================================================================
--- amd64/amd64/vm_machdep.c	(revision 195227)
+++ amd64/amd64/vm_machdep.c	(working copy)
@@ -171,6 +171,7 @@
 	pcb2->pcb_rsp = (register_t)td2->td_frame - sizeof(void *);
 	pcb2->pcb_rbx = (register_t)td2;		/* fork_trampoline argument */
 	pcb2->pcb_rip = (register_t)fork_trampoline;
+	pcb2->pcb_flags &= ~PCB_SEGLOADED;
 	/*-
 	 * pcb2->pcb_dr*:	cloned above.
 	 * pcb2->pcb_savefpu:	cloned above.
@@ -335,7 +336,7 @@
 	 * values here.
 	 */
 	bcopy(td0->td_pcb, pcb2, sizeof(*pcb2));
-	pcb2->pcb_flags &= ~PCB_FPUINITDONE;
+	pcb2->pcb_flags &= ~(PCB_FPUINITDONE | PCB_SEGLOADED);
 
 	/*
 	 * Create a new fresh stack for the new thread.
@@ -428,6 +429,7 @@
 	td->td_frame->tf_fs = _ufssel;
 	td->td_frame->tf_gs = _ugssel;
 	td->td_frame->tf_flags = TF_HASSEGS;
+	td->td_pcb->pcb_flags &= ~PCB_SEGLOADED;
 
 	/*
 	 * Pass the address of the mailbox for this kse to the uts
@@ -450,6 +452,8 @@
 	}
 #endif
 	td->td_pcb->pcb_fsbase = (register_t)tls_base;
+	td->td_pcb->pcb_flags &= ~PCB_SEGLOADED;
+
 	return (0);
 }
 
Index: amd64/amd64/exception.S
===================================================================
--- amd64/amd64/exception.S	(revision 195227)
+++ amd64/amd64/exception.S	(working copy)
@@ -635,6 +635,9 @@
 	movq	PCPU(CURTHREAD),%r8
 	movq	TD_PCB(%r8),%r8
 
+	testl	$PCB_SEGLOADED,PCB_FLAGS(%r8)
+	je	ld_regs
+
 	/*
 	 * Do not reload segment registers for kernel.
 	 * Since we do not reload segments registers with sane
@@ -650,6 +653,7 @@
 
 do_segs:
 	/* Restore %fs and fsbase */
+	andl	$PCB_SEGLOADED,PCB_FLAGS(%r8)
 	movw	TF_FS(%rsp),%ax
 	.globl	ld_fs
 ld_fs:	movw	%ax,%fs
Index: amd64/amd64/genassym.c
===================================================================
--- amd64/amd64/genassym.c	(revision 195227)
+++ amd64/amd64/genassym.c	(working copy)
@@ -64,7 +64,6 @@
 #include <net/if.h>
 #include <netinet/in.h>
 #include <nfs/nfsproto.h>
-#include <nfs/rpcv2.h>
 #include <nfsclient/nfs.h>
 #include <nfsclient/nfsdiskless.h>
 #include <machine/apicreg.h>
@@ -142,6 +141,7 @@
 ASSYM(PCB_DR6, offsetof(struct pcb, pcb_dr6));
 ASSYM(PCB_DR7, offsetof(struct pcb, pcb_dr7));
 ASSYM(PCB_TSSP, offsetof(struct pcb, pcb_tssp));
+ASSYM(PCB_SEGLOADED, PCB_SEGLOADED);
 ASSYM(PCB_DBREGS, PCB_DBREGS);
 ASSYM(PCB_32BIT, PCB_32BIT);
 ASSYM(PCB_GS32BIT, PCB_GS32BIT);
Index: amd64/amd64/cpu_switch.S
===================================================================
--- amd64/amd64/cpu_switch.S	(revision 195227)
+++ amd64/amd64/cpu_switch.S	(working copy)
@@ -97,6 +97,7 @@
 ENTRY(cpu_switch)
 	/* Switch to new thread.  First, save context. */
 	movq	TD_PCB(%rdi),%r8
+	andl	$~PCB_SEGLOADED,PCB_FLAGS(%r8)
 
 	movq	(%rsp),%rax			/* Hardware registers */
 	movq	%r15,PCB_R15(%r8)
Index: amd64/amd64/sys_machdep.c
===================================================================
--- amd64/amd64/sys_machdep.c	(revision 195227)
+++ amd64/amd64/sys_machdep.c	(working copy)
@@ -164,6 +164,8 @@
 	uint64_t a64base;
 	struct i386_ioperm_args iargs;
 
+	pcb->pcb_flags &= ~PCB_SEGLOADED;
+
 	if (uap->op == I386_GET_LDT || uap->op == I386_SET_LDT)
 		return (sysarch_ldt(td, uap, UIO_USERSPACE));
 	/*
Index: amd64/amd64/machdep.c
===================================================================
--- amd64/amd64/machdep.c	(revision 195227)
+++ amd64/amd64/machdep.c	(working copy)
@@ -372,6 +372,7 @@
 		sigexit(td, SIGILL);
 	}
 
+	td->td_pcb->pcb_flags &= ~PCB_SEGLOADED;
 	regs->tf_rsp = (long)sfp;
 	regs->tf_rip = PS_STRINGS - *(p->p_sysent->sv_szsigcode);
 	regs->tf_rflags &= ~(PSL_T | PSL_D);
@@ -850,7 +851,7 @@
 	
 	pcb->pcb_fsbase = 0;
 	pcb->pcb_gsbase = 0;
-	pcb->pcb_flags &= ~(PCB_32BIT | PCB_GS32BIT);
+	pcb->pcb_flags &= ~(PCB_SEGLOADED | PCB_32BIT | PCB_GS32BIT);
 	pcb->pcb_initial_fpucw = __INITIAL_FPUCW__;
 
 	bzero((char *)regs, sizeof(struct trapframe));
@@ -2006,6 +2007,7 @@
 		td->td_pcb->pcb_gsbase = mcp->mc_gsbase;
 	}
 	td->td_pcb->pcb_flags |= PCB_FULLCTX;
+	td->td_pcb->pcb_flags &= ~PCB_SEGLOADED;
 	return (0);
 }
 
Index: amd64/amd64/trap.c
===================================================================
--- amd64/amd64/trap.c	(revision 195227)
+++ amd64/amd64/trap.c	(working copy)
@@ -512,6 +512,7 @@
 			if (td->td_intr_nesting_level != 0)
 				break;
 
+			td->td_pcb->pcb_flags &= ~PCB_SEGLOADED;
 			/*
 			 * Invalid segment selectors and out of bounds
 			 * %rip's and %rsp's can be set up in user mode.
Index: amd64/ia32/ia32_reg.c
===================================================================
--- amd64/ia32/ia32_reg.c	(revision 195227)
+++ amd64/ia32/ia32_reg.c	(working copy)
@@ -138,6 +138,7 @@
 	tp->tf_rflags = regs->r_eflags;
 	tp->tf_rsp = regs->r_esp;
 	tp->tf_ss = regs->r_ss;
+	pcb->pcb_flags &= ~PCB_SEGLOADED;
 	return (0);
 }