Some clarifications and updates for the comments, mostly retrieved

from Bruce Evans.  Trim the trailing spaces.

MFC after:	1 week

sys/amd64/amd64/exception.S

@@ -88,10 +88,14 @@ dtrace_invop_calltrap_addr:
  * error code for those traps where the cpu doesn't in order to maintain
  * a consistent frame.  We also push a contrived 'trap number'.
  *
- * The cpu does not push the general registers, we must do that, and we 
- * must restore them prior to calling 'iret'.  The cpu adjusts the %cs and
- * %ss segment registers, but does not mess with %ds, %es, or %fs.  Thus we
- * must load them with appropriate values for supervisor mode operation.
+ * The CPU does not push the general registers, so we must do that, and we
+ * must restore them prior to calling 'iret'.  The CPU adjusts %cs and %ss
+ * but does not mess with %ds, %es, %gs or %fs.  We swap the %gs base for
+ * for the kernel mode operation shortly, without changes to the selector
+ * loaded.  Since superuser long mode works with any selectors loaded into
+ * segment registers other then %cs, which makes them mostly unused in long
+ * mode, and kernel does not reference %fs, leave them alone.  The segment
+ * registers are reloaded on return to the usermode.
  */
 
 MCOUNT_LABEL(user)

sys/i386/i386/exception.s

@@ -145,13 +145,14 @@ IDTVEC(xmm)
 	pushl $0; TRAP(T_XMMFLT)
 
 	/*
-	 * alltraps entry point.  Interrupts are enabled if this was a trap
-	 * gate (TGT), else disabled if this was an interrupt gate (IGT).
-	 * Note that int0x80_syscall is a trap gate.   Interrupt gates are
-	 * used by page faults, non-maskable interrupts, debug and breakpoint
+	 * All traps except ones for syscalls jump to alltraps.  If
+	 * interrupts were enabled when the trap occurred, then interrupts
+	 * are enabled now if the trap was through a trap gate, else
+	 * disabled if the trap was through an interrupt gate.  Note that
+	 * int0x80_syscall is a trap gate.   Interrupt gates are used by
+	 * page faults, non-maskable interrupts, debug and breakpoint
 	 * exceptions.
 	 */
-
 	SUPERALIGN_TEXT
 	.globl	alltraps
 	.type	alltraps,@function
@@ -215,10 +216,10 @@ norm_ill:
 #endif
 
 /*
- * SYSCALL CALL GATE (old entry point for a.out binaries)
+ * Call gate entry for syscalls (lcall 7,0).
+ * This is used by FreeBSD 1.x a.out executables and "old" NetBSD executables.
  *
  * The intersegment call has been set up to specify one dummy parameter.
- *
  * This leaves a place to put eflags so that the call frame can be
  * converted to a trap frame. Note that the eflags is (semi-)bogusly
  * pushed into (what will be) tf_err and then copied later into the
@@ -246,11 +247,13 @@ IDTVEC(lcall_syscall)
 	jmp	doreti
 
 /*
- * Call gate entry for FreeBSD ELF and Linux/NetBSD syscall (int 0x80)
+ * Trap gate entry for syscalls (int 0x80).
+ * This is used by FreeBSD ELF executables, "new" NetBSD executables, and all
+ * Linux executables.
  *
- * Even though the name says 'int0x80', this is actually a TGT (trap gate)
- * rather then an IGT (interrupt gate).  Thus interrupts are enabled on
- * entry just as they are for a normal syscall.
+ * Even though the name says 'int0x80', this is actually a trap gate, not an
+ * interrupt gate.  Thus interrupts are enabled on entry just as they are for
+ * a normal syscall.
  */
 	SUPERALIGN_TEXT
 IDTVEC(int0x80_syscall)
@@ -348,12 +351,13 @@ doreti_next:
 	/*
 	 * PSL_VM must be checked first since segment registers only
 	 * have an RPL in non-VM86 mode.
+	 * ASTs can not be handled now if we are in a vm86 call.
 	 */
-	testl	$PSL_VM,TF_EFLAGS(%esp)	/* are we in vm86 mode? */
+	testl	$PSL_VM,TF_EFLAGS(%esp)
 	jz	doreti_notvm86
 	movl	PCPU(CURPCB),%ecx
-	testl	$PCB_VM86CALL,PCB_FLAGS(%ecx)	/* are we in a vm86 call? */
-	jz	doreti_ast		/* can handle ASTS now if not */
+	testl	$PCB_VM86CALL,PCB_FLAGS(%ecx)
+	jz	doreti_ast
 	jmp	doreti_exit
 
 doreti_notvm86: