source: mainline/kernel/arch/mips32/src/start.S@ d7ef14b

#
# Copyright (c) 2003-2004 Jakub Jermar
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
#
# - Redistributions of source code must retain the above copyright
#   notice, this list of conditions and the following disclaimer.
# - Redistributions in binary form must reproduce the above copyright
#   notice, this list of conditions and the following disclaimer in the
#   documentation and/or other materials provided with the distribution.
# - The name of the author may not be used to endorse or promote products
#   derived from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
# OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
# IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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#include <arch/asm/regname.h>
#include <arch/mm/page.h>
#include <arch/asm/boot.h>
#include <arch/context_offset.h>
#include <arch/stack.h>

.text

.set noat
.set noreorder
.set nomacro

.global kernel_image_start
.global tlb_refill_entry
.global cache_error_entry
.global exception_entry
.global userspace_asm

/*
 * Which status bits should are thread-local:
 * KSU(UM), EXL, ERL, IE
 */
#define REG_SAVE_MASK 0x1f

/*
 * Save registers to space defined by \r
 * We will change status: Disable ERL, EXL, UM, IE
 * These changes will be automatically reversed in REGISTER_LOAD
 * %sp is NOT saved as part of these registers
 */
.macro REGISTERS_STORE_AND_EXC_RESET r
        sw $at, EOFFSET_AT(\r)
        sw $v0, EOFFSET_V0(\r)
        sw $v1, EOFFSET_V1(\r)
        sw $a0, EOFFSET_A0(\r)
        sw $a1, EOFFSET_A1(\r)
        sw $a2, EOFFSET_A2(\r)
        sw $a3, EOFFSET_A3(\r)
        sw $t0, EOFFSET_T0(\r)
        sw $t1, EOFFSET_T1(\r)
        sw $t2, EOFFSET_T2(\r)
        sw $t3, EOFFSET_T3(\r)
        sw $t4, EOFFSET_T4(\r)
        sw $t5, EOFFSET_T5(\r)
        sw $t6, EOFFSET_T6(\r)
        sw $t7, EOFFSET_T7(\r)
        sw $t8, EOFFSET_T8(\r)
        sw $t9, EOFFSET_T9(\r)
        
        mflo $at
        sw $at, EOFFSET_LO(\r)
        mfhi $at
        sw $at, EOFFSET_HI(\r)
        
        sw $gp, EOFFSET_GP(\r)
        sw $ra, EOFFSET_RA(\r)
        sw $k1, EOFFSET_K1(\r)
        
        mfc0 $t0, $status
        mfc0 $t1, $epc
        
        /* save only KSU, EXL, ERL, IE */
        and $t2, $t0, REG_SAVE_MASK
        
        /* clear KSU, EXL, ERL, IE */
        li $t3, ~(REG_SAVE_MASK)
        and $t0, $t0, $t3
        
        sw $t2, EOFFSET_STATUS(\r)
        sw $t1, EOFFSET_EPC(\r)
        mtc0 $t0, $status
.endm

.macro REGISTERS_LOAD r
        /*
         * Update only UM, EXR, IE from status, the rest
         * is controlled by OS and not bound to task.
         */
        mfc0 $t0, $status
        lw $t1,EOFFSET_STATUS(\r)
        
        /* mask UM, EXL, ERL, IE */
        li $t2, ~REG_SAVE_MASK
        and $t0, $t0, $t2
        
        /* copy UM, EXL, ERL, IE from saved status */
        or $t0, $t0, $t1
        mtc0 $t0, $status
        
        lw $v0, EOFFSET_V0(\r)
        lw $v1, EOFFSET_V1(\r)
        lw $a0, EOFFSET_A0(\r)
        lw $a1, EOFFSET_A1(\r)
        lw $a2, EOFFSET_A2(\r)
        lw $a3, EOFFSET_A3(\r)
        lw $t0, EOFFSET_T0(\r)
        lw $t1, EOFFSET_T1(\r)
        lw $t2, EOFFSET_T2(\r)
        lw $t3, EOFFSET_T3(\r)
        lw $t4, EOFFSET_T4(\r)
        lw $t5, EOFFSET_T5(\r)
        lw $t6, EOFFSET_T6(\r)
        lw $t7, EOFFSET_T7(\r)
        lw $t8, EOFFSET_T8(\r)
        lw $t9, EOFFSET_T9(\r)
        
        lw $gp, EOFFSET_GP(\r)
        lw $ra, EOFFSET_RA(\r)
        lw $k1, EOFFSET_K1(\r)
        
        lw $at, EOFFSET_LO(\r)
        mtlo $at
        lw $at, EOFFSET_HI(\r)
        mthi $at
        
        lw $at, EOFFSET_EPC(\r)
        mtc0 $at, $epc
        
        lw $at, EOFFSET_AT(\r)
        lw $sp, EOFFSET_SP(\r)
.endm

/*
 * Move kernel stack pointer address to register $k0.
 * If we are in user mode, load the appropriate stack address.
 */
.macro KERNEL_STACK_TO_K0
        /* if we are in user mode */
        mfc0 $k0, $status
        andi $k0, 0x10
        
        beq $k0, $0, 1f
        add $k0, $sp, 0
        
        /* move $k0 pointer to kernel stack */
        lui $k0, %hi(supervisor_sp)
        ori $k0, $k0, %lo(supervisor_sp)
        
        /* move $k0 (supervisor_sp) */
        lw $k0, 0($k0)
        
        1:
.endm

.org 0x0
kernel_image_start:
        /* load temporary stack */
        lui $sp, %hi(end_stack)
        ori $sp, $sp, %lo(end_stack)
        
        /* not sure about this, but might be needed for PIC code */
        lui $gp, 0x8000
        
        /* $a1 contains physical address of bootinfo_t */
        jal arch_pre_main
        nop
        
        j main_bsp
        nop

.space TEMP_STACK_SIZE
end_stack:

tlb_refill_entry:
        j tlb_refill_handler
        nop

cache_error_entry:
        j cache_error_handler
        nop

exception_entry:
        j exception_handler
        nop

exception_handler:
        KERNEL_STACK_TO_K0
        
        sub $k0, REGISTER_SPACE
        sw $sp, EOFFSET_SP($k0)
        move $sp, $k0
        
        mfc0 $k0, $cause
        
        sra $k0, $k0, 0x2    /* cp0_exc_cause() part 1 */
        andi $k0, $k0, 0x1f  /* cp0_exc_cause() part 2 */
        sub $k0, 8           /* 8 = SYSCALL */
        
        beqz $k0, syscall_shortcut
        add $k0, 8           /* revert $k0 back to correct exc number */
        
        REGISTERS_STORE_AND_EXC_RESET $sp
        
        move $a1, $sp
        jal exc_dispatch     /* exc_dispatch(excno, register_space) */
        move $a0, $k0
        
        REGISTERS_LOAD $sp
        /* the $sp is automatically restored to former value */
        eret

#define SS_SP      EOFFSET_SP
#define SS_STATUS  EOFFSET_STATUS
#define SS_EPC     EOFFSET_EPC
#define SS_K1      EOFFSET_K1

/** Syscall entry
 *
 * Registers:
 *
 * @param $v0 Syscall number.
 * @param $a0 1st argument.
 * @param $a1 2nd argument.
 * @param $a2 3rd argument.
 * @param $a3 4th argument.
 * @param $t0 5th argument.
 * @param $t1 6th argument.
 *
 * @return The return value will be stored in $v0.
 *
 */
syscall_shortcut:
        /* we have a lot of space on the stack, with free use */
        mfc0 $t3, $epc
        mfc0 $t2, $status
        sw $t3, SS_EPC($sp)  /* save EPC */
        sw $k1, SS_K1($sp)   /* save $k1 not saved on context switch */
        
        and $t4, $t2, REG_SAVE_MASK  /* save only KSU, EXL, ERL, IE */
        li $t5, ~(0x1f)
        and $t2, $t2, $t5  /* clear KSU, EXL, ERL */
        ori $t2, $t2, 0x1  /* set IE */
        
        sw $t4, SS_STATUS($sp)
        mtc0 $t2, $status
        
        /*
         * Call the higher level system call handler.
         * We are going to reuse part of the unused exception stack frame.
         *
         */
        sw $t0, STACK_ARG4($sp)  /* save the 5th argument on the stack */
        sw $t1, STACK_ARG5($sp)  /* save the 6th argument on the stack */
        jal syscall_handler
        sw $v0, STACK_ARG6($sp)  /* save the syscall number on the stack */
        
        /* restore status */
        mfc0 $t2, $status
        lw $t3, SS_STATUS($sp)
        
        /*
         * Change back to EXL = 1 (from last exception), otherwise
         * an interrupt could rewrite the CP0 - EPC.
         *
         */
        li $t4, ~REG_SAVE_MASK  /* mask UM, EXL, ERL, IE */
        and $t2, $t2, $t4
        or $t2, $t2, $t3  /* copy saved UM, EXL, ERL, IE */
        mtc0 $t2, $status
        
        /* restore epc + 4 */
        lw $t2, SS_EPC($sp)
        lw $k1, SS_K1($sp)
        addi $t2, $t2, 4
        mtc0 $t2, $epc
        
        lw $sp, SS_SP($sp)  /* restore $sp */
        eret

tlb_refill_handler:
        KERNEL_STACK_TO_K0
        sub $k0, REGISTER_SPACE
        REGISTERS_STORE_AND_EXC_RESET $k0
        sw $sp,EOFFSET_SP($k0)
        add $sp, $k0, 0
        
        jal tlb_refill
        add $a0, $sp, 0 
        
        REGISTERS_LOAD $sp
        eret

cache_error_handler:
        KERNEL_STACK_TO_K0
        sub $k0, REGISTER_SPACE
        REGISTERS_STORE_AND_EXC_RESET $k0
        sw $sp,EOFFSET_SP($k0)
        add $sp, $k0, 0
        
        jal cache_error
        add $a0, $sp, 0
        
        REGISTERS_LOAD $sp
        eret

userspace_asm:
        add $sp, $a0, 0
        add $v0, $a1, 0
        add $t9, $a2, 0    /* set up correct entry into PIC code */
        xor $a0, $a0, $a0  /* $a0 is defined to hold pcb_ptr */
                           /* set it to 0 */
        eret