[MIPS] Implementation of sel_ldr for MIPS architecture.

src/trusted/service_runtime/arch/mips/tramp_mips.S

+/*
+ * Copyright 2012 The Native Client Authors.  All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can
+ * be found in the LICENSE file.
+ */
+
+#include "native_client/src/trusted/service_runtime/nacl_config.h"
+
+.text
+
+/*
+ * Assembly code template.
+ * This is linked into the service runtime but is unused as code -- it is used
+ * as data to be patched into a NaCl app's address space.
+ *
+ * Trampoline to transfer control from native client module to
+ * sel_ldr's NaClSyscallSeg residing in the service runtime portion of address
+ * space. Trampolines are patched into nacl module's address space in the
+ * trampoline region. They are patched by NaClLoadTrampoline() code (sel_ldr.c).
+ * Each trampoline code segment corresponds to a system call, so the trampoline
+ * region is full of identical trampoline code segments. Service runtime
+ * distinguish which system call is requested using the address of an executed
+ * trampoline (it is saved on stack in NaClSyscallSeg()).
+ *
+ * The trampoline code should push first 4 parameters of the system call to
+ * the stack, and on top of that it should push the return address. On Mips the
+ * trampoline is too small for all this, so the only thing that trampoline does
+ * is call the function NaClSyscallSeg where the push is actually done.
+ */
+
+DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaCl_trampoline_seg_code):
+
+  .set noreorder
+
+  /* We don't actually load $t9 with zero, but we patch it with real address

[Mark Seaborn, 2012/09/18 03:24:34]

Nit: please use the NaCl comment style of
/*
 * ...
 */
(this does apply in some other cases in your change too)

[petarj, 2012/09/19 17:27:51]

Done.

+   * of NaClSyscallSeg during placement of trampoline code. */
+
+  lui   $t9, 0
+  ori   $t9, $t9, 0
+  jalr  $t5, $t9
+  nop
+
+  jr $0 /* NACL_HALT - at the start of the second block of the trampoline */
+  .set reorder
+
+DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaCl_trampoline_seg_end):
+