[MIPS] Restore the trusted stack in NaClSyscallSeg.

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

Index: src/trusted/service_runtime/arch/mips/nacl_syscall.S
diff --git a/src/trusted/service_runtime/arch/mips/nacl_syscall.S b/src/trusted/service_runtime/arch/mips/nacl_syscall.S
index 9fc351f1970e73af89c17d9ba332a47d889f4b71..0841ac5e5965d1b290f5867231406fdcb369dd32 100644
--- a/src/trusted/service_runtime/arch/mips/nacl_syscall.S
+++ b/src/trusted/service_runtime/arch/mips/nacl_syscall.S
@@ -113,8 +113,9 @@ DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaClSyscallSeg):
   /* Restore the thread index. */
   srl   $a0, $a0, 2
 
-  /* Reserve 16-byte for arguments (o32 ABI) */

[Mark Seaborn, 2012/11/01 01:47:41]

Should you change NaClStartThreadInApp() to explicitly reserve these 16 bytes? 
We used to reserve 32 bytes that way for the Windows x86-64 ABI (see before
http://src.chromium.org/viewvc/native_client?view=rev&revision=7168).

Currently it does this for MIPS:

  natp->user.trusted_stack_ptr = (NaClGetStackPtr() & ~0xf) + 4;

The "+ 4" there is inconsistent with NACL_STACK_PAD_BELOW_ALIGN in
nacl_config.h, BTW.

[petarj, 2012/11/01 17:06:40]

The 16 bytes are already reserved at that point. No need to explicitly reserve
them.
The inconsistency has been corrected.

[Mark Seaborn, 2012/11/01 18:01:57]

OK, good point.

-  addiu $sp, $sp, -16
+  /* Restore the trusted stack */
+  lw $sp, NACL_THREAD_CONTEXT_OFFSET_TRUSTED_STACK_PTR($a1)
+
   lw    $t9,%call16(NaClSyscallCSegHook)($gp)
   jr    $t9
   nop