x86-64: merge the standard and compat start_thread() functions

The only thing left that differs between the standard and compat
start_thread functions is the actual segment numbers and the
prototype, so have a single common function which contains the guts
and two very small wrappers.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>

arch/x86/kernel/process_64.c

@@ -344,18 +344,20 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 	return err;
 }
 
-void
-start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
+static void
+start_thread_common(struct pt_regs *regs, unsigned long new_ip,
+		    unsigned long new_sp,
+		    unsigned int _cs, unsigned int _ss, unsigned int _ds)
 {
 	loadsegment(fs, 0);
-	loadsegment(es, 0);
-	loadsegment(ds, 0);
+	loadsegment(es, _ds);
+	loadsegment(ds, _ds);
 	load_gs_index(0);
 	regs->ip		= new_ip;
 	regs->sp		= new_sp;
 	percpu_write(old_rsp, new_sp);
-	regs->cs		= __USER_CS;
-	regs->ss		= __USER_DS;
+	regs->cs		= _cs;
+	regs->ss		= _ss;
 	regs->flags		= X86_EFLAGS_IF;
 	set_fs(USER_DS);
 	/*
@@ -363,26 +365,19 @@ start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
 	 */
 	free_thread_xstate(current);
 }
-EXPORT_SYMBOL_GPL(start_thread);
+
+void
+start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
+{
+	start_thread_common(regs, new_ip, new_sp,
+			    __USER_CS, __USER_DS, 0);
+}
 
 #ifdef CONFIG_IA32_EMULATION
 void start_thread_ia32(struct pt_regs *regs, u32 new_ip, u32 new_sp)
 {
-	loadsegment(fs, 0);
-	loadsegment(ds, __USER32_DS);
-	loadsegment(es, __USER32_DS);
-	load_gs_index(0);
-	regs->ip		= new_ip;
-	regs->sp		= new_sp;
-	percpu_write(old_rsp, new_sp);
-	regs->cs		= __USER32_CS;
-	regs->ss		= __USER32_DS;
-	regs->flags		= X86_EFLAGS_IF;
-	set_fs(USER_DS);
-	/*
-	 * Free the old FP and other extended state
-	 */
-	free_thread_xstate(current);
+	start_thread_common(regs, new_ip, new_sp,
+			    __USER32_CS, __USER32_DS, __USER32_DS);
 }
 #endif