Index: arch/amd64/src/boot/boot.S
===================================================================
--- arch/amd64/src/boot/boot.S	(revision 37b451f7a8c252d1c0a3c5b78efd144d682378ec)
+++ arch/amd64/src/boot/boot.S	(revision 65640fefbb56b135a33b91d53c6b87fd88b9f440)
@@ -59,8 +59,11 @@
 	movw $gdtselector(KDATA_DES), %cx
 	movw %cx, %es
-	movw %cx, %gs
-	movw %cx, %fs
 	movw %cx, %ds							# kernel data + stack
 	movw %cx, %ss
+	# Simics seems to remove hidden part of GS on entering user mode
+	#  when _visible_ part of GS does not point to user-mode segment
+	movw $gdtselector(UDATA_DES), %cx
+	movw %cx, %fs
+	movw %cx, %gs
 	
 	jmpl $gdtselector(KTEXT32_DES), $multiboot_meeting_point