Make the default values for DS/ES be the _user_ segment descriptors

on x86 - the kernel doesn't really care (as long as it's all flat 32-bit),
and it means that the return path for sysenter/sysexit can avoid re-loading
the segment registers.

NOTE! This means that _all_ kernel code (not just the sysenter path) must
be appropriately changed, since the kernel knows the conventions and doesn't
save/restore DS/ES internally on context switches etc.

arch/i386/kernel/entry.S

@@ -91,18 +91,21 @@ VM_MASK		= 0x00020000
 	pushl %edx; \
 	pushl %ecx; \
 	pushl %ebx; \
-	movl $(__KERNEL_DS), %edx; \
+	movl $(__USER_DS), %edx; \
 	movl %edx, %ds; \
 	movl %edx, %es;
 
-#define RESTORE_REGS	\
+#define RESTORE_INT_REGS \
 	popl %ebx;	\
 	popl %ecx;	\
 	popl %edx;	\
 	popl %esi;	\
 	popl %edi;	\
 	popl %ebp;	\
-	popl %eax;	\
+	popl %eax
+
+#define RESTORE_REGS	\
+	RESTORE_INT_REGS; \
 1:	popl %ds;	\
 2:	popl %es;	\
 .section .fixup,"ax";	\
@@ -271,9 +274,9 @@ ENTRY(sysenter_entry)
 	movl TI_FLAGS(%ebx), %ecx
 	testw $_TIF_ALLWORK_MASK, %cx
 	jne syscall_exit_work
-	RESTORE_REGS
-	movl 4(%esp),%edx
-	movl 16(%esp),%ecx
+	RESTORE_INT_REGS
+	movl 12(%esp),%edx
+	movl 24(%esp),%ecx
 	sti
 	sysexit
 
@@ -428,7 +431,7 @@ error_code:
 	movl %esp, %edx
 	pushl %esi			# push the error code
 	pushl %edx			# push the pt_regs pointer
-	movl $(__KERNEL_DS), %edx
+	movl $(__USER_DS), %edx
 	movl %edx, %ds
 	movl %edx, %es
 	call *%edi

arch/i386/kernel/head.S

@@ -235,12 +235,15 @@ is386:	movl $2,%ecx		# set MP
 	lidt idt_descr
 	ljmp $(__KERNEL_CS),$1f
 1:	movl $(__KERNEL_DS),%eax	# reload all the segment registers
-	movl %eax,%ds		# after changing gdt.
+	movl %eax,%ss			# after changing gdt.
+
+	movl $(__USER_DS),%eax		# DS/ES contains default USER segment
+	movl %eax,%ds
 	movl %eax,%es
+
+	xorl %eax,%eax			# Clear FS/GS and LDT
 	movl %eax,%fs
 	movl %eax,%gs
-	movl %eax,%ss
-	xorl %eax,%eax
 	lldt %ax
 	cld			# gcc2 wants the direction flag cleared at all times
 #ifdef CONFIG_SMP

arch/i386/kernel/process.c

@@ -219,8 +219,8 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 	regs.ebx = (unsigned long) fn;
 	regs.edx = (unsigned long) arg;
 
-	regs.xds = __KERNEL_DS;
-	regs.xes = __KERNEL_DS;
+	regs.xds = __USER_DS;
+	regs.xes = __USER_DS;
 	regs.orig_eax = -1;
 	regs.eip = (unsigned long) kernel_thread_helper;
 	regs.xcs = __KERNEL_CS;