[MIPS] Implementation of sel_ldr for MIPS architecture.

src/trusted/service_runtime/arch/mips/nacl_switch.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
+
+/*
+ * This trusted code is linked into the service_runtime and
+ * executed when switching from the service runtime to a nacl
+ * module. This happens when a main nacl thread is created and starting to
+ * execute the nacl code, or when nacl module is returning from a system
+ * call. This piece of code lives in a service runtime part of address space.
+ * The one and only argument is in register a0
+ *
+ * a0 -- address of thread context (struct NaClThreadContext)
+ */
+
+DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaClSwitch):
+
+  .set noreorder
+
+/*
+ * We clear registers a1, a2, a3, t0, t1, t2, t3, t4, t5 and ra to avoid
+ * information leaks. The remaining registers are overwritten by the code that
+ * follows after.
+ */
+
+  addu $a1, $zero, $zero
+  addu $a2, $zero, $zero
+  addu $a3, $zero, $zero
+  addu $t0, $zero, $zero
+  addu $t1, $zero, $zero
+  addu $t2, $zero, $zero
+  addu $t3, $zero, $zero
+  addu $t4, $zero, $zero
+  addu $t5, $zero, $zero
+  addu $ra, $zero, $zero
+
+  /* NACL_CALLEE_SAVE_LIST BEGIN */
+  lw $t6, 0($a0)
+  lw $t7, 4($a0)
+  lw $s0, 8($a0)
+  lw $s1, 12($a0)
+  lw $s2, 16($a0)
+  lw $s3, 20($a0)
+  lw $s4, 24($a0)
+  lw $s5, 28($a0)
+  lw $s6, 32($a0)
+  lw $s7, 36($a0)
+  lw $t8, 40($a0)
+  lw $gp, 44($a0)
+  lw $sp, 48($a0)
+  lw $fp, 52($a0)
+  /* NACL_CALLEE_SAVE_LIST END*/
+
+  lw $v0, 60($a0)      /* context->sysret       */
+  lw $v1, 64($a0)      /* context->new_prog_ctr */
+
+  /* At startup, context->sysret contains not the the return value, but the
+     first argument. Put it in a0. */
+  addu $a0, $v0, $zero
+
+  /* Transfer control to untrusted code */
+  jr  $v1
+  nop
+
+  .set reorder
+