[MIPS] Implementation of sel_ldr for MIPS architecture.

src/trusted/service_runtime/arch/mips/nacl_syscall.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/arch/mips/sel_rt.h"
+#include "native_client/src/trusted/service_runtime/nacl_config.h"
+
+.text
+
+
+/*
+ * This trusted code is linked into the service runtime. It is executed when a
+ * nacl module performs a system call via a service runtime interface. The nacl
+ * module jumps to the trampoline corresponding to the syscall and then here.
+ * This code switches the execution contexts (registers and stack) from
+ * untrusted to trusted.
+ *
+ * on stack:
+ *
+ * top
+ * =====
+ * a0 - alignment for NaClSyscallCSegHook
+ * ra - return address
+ * arg 1
+ * arg 2
+ * .....
+ * arg N
+ */
+
+DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaClSyscallSeg):
+
+  .set noreorder
+
+/*
+ * The following code (next 6 instructions) does what the trampoline code should
+ * do. It places the first 4 parameters of the system call to the stack, and on
+ * top of that it places the return address to the user module. On Mips the
+ * trampoline is too small for all this, so the code is moved here.
+ *
+ * Mips passes parameters to a callee in registers a0-a3. If there are more
+ * than 4 parameters, the first four args are passed in registers and the rest
+ * are placed on the stack. Regardless of the number of arguments passed, Mips
+ * o32 ABI requires that the caller should always reserve 16 bytes on stack that
+ * correspond to registers a0-a3.
+ *
+ * This code pushes all parameters from registers into the stack; thus, we keep
+ * all parameters on the stack as follows:
+ * top - arg0, arg1, arg2, arg3 .... argN
+ *
+ * On top of that we push the return address, so we will know where to return
+ * after the system call.
+ *
+ * After this code the stack layout must look like:
+ *
+ * sp+0: retaddr to user module
+ * sp+4: arg0 (system call argument)
+ * sp+8: arg1
+ * ..........
+ * sp+X: argN
+ *
+ * When service runtime serves a system call, it first creates a structure which
+ * utilizes these arguments. The structure is created by Decoder functions in
+ * nacl_syscall_handlers.c. (nacl_syscall_handlers.c is an automatically
+ * generated file and placed in
+ * scons-out//gen/native_client/src/trusted/service_runtime).
+ */
+
+  /* Place a0-a3 argument registers in the reserved 16-byte region in caller's
+     stack frame (o32 ABI), and place return address on top of that. */
+
+  .cpload $t9
+
+  sw    $a3, 12($sp)
+  sw    $a2,  8($sp)
+  sw    $a1,  4($sp)
+  sw    $a0,  0($sp)
+  addiu $sp, $sp, -8
+  sw    $ra,  4($sp)
+  sw    $t5,  0($sp)
+
+  /* Load the __thread variable's offset into a3. */
+  lui   $a3, %tprel_hi(gNaClThreadIdx)
+  addiu $a3, $a3, %tprel_lo(gNaClThreadIdx)
+
+  /* Fetch the thread-local variable holding the thread index. */
+  rdhwr $v1, $29
+  addu  $a3, $v1, $a3
+  lw    $a0, 0($a3)
+
+  /* Load the nacl_user entry of this thread.
+   * a1 = nacl_user[a0*4] */
+  lw    $a1,%got(nacl_user)($28)
+  sll   $a0, $a0, 2
+  addu  $a1, $a1, $a0
+  lw    $a1, 0($a1)
+
+  sw $t6, NACL_THREAD_CONTEXT_OFFSET_T6($a1)

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

Optional: if t6/t7/t8 are constants, a micro-optimisation here would be not to
save them, right?  (Either way, you could comment this.)

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

t6 and 67 are constants. The optimization is actually tiny, since MIPS needs two
instructions to load an arbitrary 32-bit constant.
It has just been done, though

+  sw $t7, NACL_THREAD_CONTEXT_OFFSET_T7($a1)
+  sw $s0, NACL_THREAD_CONTEXT_OFFSET_S0($a1)
+  sw $s1, NACL_THREAD_CONTEXT_OFFSET_S1($a1)
+  sw $s2, NACL_THREAD_CONTEXT_OFFSET_S2($a1)
+  sw $s3, NACL_THREAD_CONTEXT_OFFSET_S3($a1)
+  sw $s4, NACL_THREAD_CONTEXT_OFFSET_S4($a1)
+  sw $s5, NACL_THREAD_CONTEXT_OFFSET_S5($a1)
+  sw $s6, NACL_THREAD_CONTEXT_OFFSET_S6($a1)
+  sw $s7, NACL_THREAD_CONTEXT_OFFSET_S7($a1)
+  sw $t8, NACL_THREAD_CONTEXT_OFFSET_T8($a1)
+  sw $gp, NACL_THREAD_CONTEXT_OFFSET_GLOBAL_PTR($a1)
+  sw $sp, NACL_THREAD_CONTEXT_OFFSET_STACK_PTR($a1)
+  sw $fp, NACL_THREAD_CONTEXT_OFFSET_FRAME_PTR($a1)
+
+  /* Restore the thread index. */
+  srl   $a0, $a0, 2
+
+  /* Reserve 16-byte for arguments (o32 ABI) */
+  addiu $sp, $sp, -16
+  lw    $t9,%call16(NaClSyscallCSegHook)($gp)
+  jr    $t9
+  nop
+
+  /* NOTREACHED */
+
+  /*
+   * If the thread returns, which must not happen, it will be halted
+   * by the following instruction
+   */
+
+  jr $0 /*NACL_HALT*/
+