target/linux/generic/patches-4.9/040-03-MIPS-Only-change-28-to-thread_info-if-coming-from-us.patch

   1 From: Matt Redfearn <matt.redfearn@imgtec.com>
   2 Date: Mon, 19 Dec 2016 14:20:58 +0000
   3 Subject: [PATCH] MIPS: Only change $28 to thread_info if coming from user
   4  mode
   5
   6 The SAVE_SOME macro is used to save the execution context on all
   7 exceptions.
   8 If an exception occurs while executing user code, the stack is switched
   9 to the kernel's stack for the current task, and register $28 is switched
  10 to point to the current_thread_info, which is at the bottom of the stack
  11 region.
  12 If the exception occurs while executing kernel code, the stack is left,
  13 and this change ensures that register $28 is not updated. This is the
  14 correct behaviour when the kernel can be executing on the separate irq
  15 stack, because the thread_info will not be at the base of it.
  16
  17 With this change, register $28 is only switched to it's kernel
  18 conventional usage of the currrent thread info pointer at the point at
  19 which execution enters kernel space. Doing it on every exception was
  20 redundant, but OK without an IRQ stack, but will be erroneous once that
  21 is introduced.
  22
  23 Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
  24 Reviewed-by: Maciej W. Rozycki <macro@imgtec.com>
  25 ---
  26
  27 --- a/arch/mips/include/asm/stackframe.h
  28 +++ b/arch/mips/include/asm/stackframe.h
  29 @@ -216,12 +216,19 @@
  30                 LONG_S  $25, PT_R25(sp)
  31                 LONG_S  $28, PT_R28(sp)
  32                 LONG_S  $31, PT_R31(sp)
  33 +
  34 +               /* Set thread_info if we're coming from user mode */
  35 +               mfc0    k0, CP0_STATUS
  36 +               sll     k0, 3           /* extract cu0 bit */
  37 +               bltz    k0, 9f
  38 +
  39                 ori     $28, sp, _THREAD_MASK
  40                 xori    $28, _THREAD_MASK
  41  #ifdef CONFIG_CPU_CAVIUM_OCTEON
  42                 .set    mips64
  43                 pref    0, 0($28)       /* Prefetch the current pointer */
  44  #endif
  45 +9:
  46                 .set    pop
  47                 .endm
  48