Don't forget to load %es with the kernel data segment selector in

Xcpustop().  %es is used in at least the call to savectx() when savectx()
calls bcopy(), so not loading it was fatal if a stop IPI interrupts
user mode.

This reduces bugs starting and stopping CPUs for debuggers.  CPUs are
stopped mainly in kdb_trap() and cpu_reset().  At reset time there is
a good chance that all the CPUs are in the kernel, so the bug was
probably harmless then.

sys/amd64/amd64/apic_vector.S

@@ -467,10 +467,12 @@ IDTVEC(cpustop)
 	pushl	%ecx
 	pushl	%edx
 	pushl	%ds			/* save current data segment */
+	pushl	%es
 	pushl	%fs
 
 	movl	$KDSEL, %eax
 	mov	%ax, %ds		/* use KERNEL data segment */
+	mov	%ax, %es
 	movl	$KPSEL, %eax
 	mov	%ax, %fs
 
@@ -507,6 +509,7 @@ IDTVEC(cpustop)
 	call	*%eax
 2:
 	popl	%fs
+	popl	%es
 	popl	%ds			/* restore previous data segment */
 	popl	%edx
 	popl	%ecx

sys/i386/i386/apic_vector.s

@@ -467,10 +467,12 @@ IDTVEC(cpustop)
 	pushl	%ecx
 	pushl	%edx
 	pushl	%ds			/* save current data segment */
+	pushl	%es
 	pushl	%fs
 
 	movl	$KDSEL, %eax
 	mov	%ax, %ds		/* use KERNEL data segment */
+	mov	%ax, %es
 	movl	$KPSEL, %eax
 	mov	%ax, %fs
 
@@ -507,6 +509,7 @@ IDTVEC(cpustop)
 	call	*%eax
 2:
 	popl	%fs
+	popl	%es
 	popl	%ds			/* restore previous data segment */
 	popl	%edx
 	popl	%ecx

sys/i386/isa/apic_vector.s

@@ -467,10 +467,12 @@ IDTVEC(cpustop)
 	pushl	%ecx
 	pushl	%edx
 	pushl	%ds			/* save current data segment */
+	pushl	%es
 	pushl	%fs
 
 	movl	$KDSEL, %eax
 	mov	%ax, %ds		/* use KERNEL data segment */
+	mov	%ax, %es
 	movl	$KPSEL, %eax
 	mov	%ax, %fs
 
@@ -507,6 +509,7 @@ IDTVEC(cpustop)
 	call	*%eax
 2:
 	popl	%fs
+	popl	%es
 	popl	%ds			/* restore previous data segment */
 	popl	%edx
 	popl	%ecx