kernel/lib/x86/longjmp.c

   1 /*
   2  *  ZeX/OS
   3  *  Copyright (C) 2007  Tomas 'ZeXx86' Jedrzejek (zexx86@zexos.org)
   4  *
   5  *  This program is free software: you can redistribute it and/or modify
   6  *  it under the terms of the GNU General Public License as published by
   7  *  the Free Software Foundation, either version 3 of the License, or
   8  *  (at your option) any later version.
   9  *
  10  *  This program is distributed in the hope that it will be useful,
  11  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  *  GNU General Public License for more details.
  14  *
  15  *  You should have received a copy of the GNU General Public License
  16  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  17  */
  18
  19
  20 #include <system.h>
  21 #include <build.h>
  22
  23 #ifdef ARCH_i386
  24
  25 /* jmp_buf */
  26 /*****************************************************************************
  27 To use setjmp() and longjmp() for asynchronous (interrupt-driven;
  28 pre-emptive) task-switching, we want to enable interrupts simultaneous
  29 with jumping to the task. In other words, we want the EFLAGS and EIP
  30 registers loaded at the same time.
  31
  32 The only instruction that can do this is IRET, which also loads the CS
  33 register. Changing CS is done in code that uses far pointers, and it's
  34 also done when changing address spaces, and when changing privilege levels.
  35 We're not interested in any of those, so just push the current CS value
  36 on the stack and let IRET use that.
  37
  38 Three distinct stack pointer (ESP) values are used in this routine:
  39 - 'Old' or 'current' stack pointer value, which is discarded by
  40   this routine (use setjmp() to save it)
  41 - ESP is made to point, briefly, to the jmp_buf struct itself
  42 - 'New' ESP value; stored in jmp_buf.esp
  43
  44 Register values are restored from the jmp_buf as follows:
  45 1. Push jmp_buf.eflags, the current CS value, and jmp_buf.eip
  46    onto the 'new' stack
  47 2. Make ESP point to the jmp_buf struct itself, then use the POPA
  48    instruction to pop the 7 general purpose registers (ESP is not
  49    loaded by POPA). The use of POPA means that registers in the
  50    jmp_buf MUST be stored in the order that POPA expects.
  51    (Maybe use MOVs instead, to eliminate this restriction?
  52    Might have to rewrite entire function in asm, instead of C.)
  53 3. Load ESP with the 'new' stack pointer, from jmp_buf.esp
  54 4. Use IRET to pop EIP, CS, and EFLAGS from the 'new' stack
  55 5. ???
  56 6. Profit!      <--- obligatory Slashdot joke
  57
  58 This code does NOT save the floating-point state of the CPU. Either:
  59 1. Don't use floating point, or
  60 2. Don't use floating point in more than one thread, or
  61 3. Rewrite this code so it DOES save the floating-point state, or
  62 4. Save/restore the floating-point state when entering/leaving
  63    the kernel (protected OS only)
  64 *****************************************************************************/
  65
  66 void longjmp(jmp_buf buf, int ret_val)
  67 {
  68         unsigned *esp;
  69
  70 /* make sure return value is not 0 */
  71         if(ret_val == 0)
  72                 ret_val++;
  73 /* EAX is used for return values, so store it in jmp_buf.EAX */
  74         buf->eax = ret_val;
  75 /* get ESP for new stack */
  76         esp = (unsigned *)buf->esp;
  77 /* push EFLAGS on the new stack */
  78         esp--;
  79         *esp = buf->eflags;
  80 /* push current CS on the new stack */
  81         esp--;
  82         __asm__ __volatile__(
  83                 "mov %%cs,%0\n"
  84                 : "=m"(*esp));
  85 /* push EIP on the new stack */
  86         esp--;
  87         *esp = buf->eip;
  88 /* new ESP is 12 bytes lower; update jmp_buf.ESP */
  89         buf->esp = (unsigned)esp;
  90
  91 /* now, briefly, make the jmp_buf struct our stack */
  92         __asm__ __volatile__(
  93                 "movl %0,%%esp\n"
  94 /* ESP now points to 8 general-purpose registers stored in jmp_buf
  95 Pop them */
  96                 "popa\n"
  97                 "sti\n"
  98 /* load new stack pointer from jmp_buf */
  99                 "movl -20(%%esp),%%esp\n"
 100 /* ESP now points to new stack, with the IRET frame (EIP, CS, EFLAGS)
 101 we created just above. Pop these registers: */
 102
 103                 "iret\n"
 104                 :
 105                 : "m"(buf));
 106 }
 107 #endif