1/* 2 NetWinder Floating Point Emulator 3 (c) Rebel.COM, 1998 4 (c) 1998, 1999 Philip Blundell 5 6 Direct questions, comments to Scott Bambrough <scottb@netwinder.org> 7 8 This program is free software; you can redistribute it and/or modify 9 it under the terms of the GNU General Public License as published by 10 the Free Software Foundation; either version 2 of the License, or 11 (at your option) any later version. 12 13 This program is distributed in the hope that it will be useful, 14 but WITHOUT ANY WARRANTY; without even the implied warranty of 15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 16 GNU General Public License for more details. 17 18 You should have received a copy of the GNU General Public License 19 along with this program; if not, write to the Free Software 20 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. 21*/ 22#include <asm/assembler.h> 23#include <asm/opcodes.h> 24 25/* This is the kernel's entry point into the floating point emulator. 26It is called from the kernel with code similar to this: 27 28 sub r4, r5, #4 29 ldrt r0, [r4] @ r0 = instruction 30 adrsvc al, r9, ret_from_exception @ r9 = normal FP return 31 adrsvc al, lr, fpundefinstr @ lr = undefined instr return 32 33 get_current_task r10 34 mov r8, #1 35 strb r8, [r10, #TSK_USED_MATH] @ set current->used_math 36 add r10, r10, #TSS_FPESAVE @ r10 = workspace 37 ldr r4, .LC2 38 ldr pc, [r4] @ Call FP emulator entry point 39 40The kernel expects the emulator to return via one of two possible 41points of return it passes to the emulator. The emulator, if 42successful in its emulation, jumps to ret_from_exception (passed in 43r9) and the kernel takes care of returning control from the trap to 44the user code. If the emulator is unable to emulate the instruction, 45it returns via _fpundefinstr (passed via lr) and the kernel halts the 46user program with a core dump. 47 48On entry to the emulator r10 points to an area of private FP workspace 49reserved in the thread structure for this process. This is where the 50emulator saves its registers across calls. The first word of this area 51is used as a flag to detect the first time a process uses floating point, 52so that the emulator startup cost can be avoided for tasks that don't 53want it. 54 55This routine does three things: 56 571) The kernel has created a struct pt_regs on the stack and saved the 58user registers into it. See /usr/include/asm/proc/ptrace.h for details. 59 602) It calls EmulateAll to emulate a floating point instruction. 61EmulateAll returns 1 if the emulation was successful, or 0 if not. 62 633) If an instruction has been emulated successfully, it looks ahead at 64the next instruction. If it is a floating point instruction, it 65executes the instruction, without returning to user space. In this 66way it repeatedly looks ahead and executes floating point instructions 67until it encounters a non floating point instruction, at which time it 68returns via _fpreturn. 69 70This is done to reduce the effect of the trap overhead on each 71floating point instructions. GCC attempts to group floating point 72instructions to allow the emulator to spread the cost of the trap over 73several floating point instructions. */ 74 75#include <asm/asm-offsets.h> 76 77 .globl nwfpe_enter 78nwfpe_enter: 79 mov r4, lr @ save the failure-return addresses 80 mov sl, sp @ we access the registers via 'sl' 81 82 ldr r5, [sp, #S_PC] @ get contents of PC; 83 mov r6, r0 @ save the opcode 84emulate: 85 ldr r1, [sp, #S_PSR] @ fetch the PSR 86 bl arm_check_condition @ check the condition 87 cmp r0, #ARM_OPCODE_CONDTEST_PASS @ condition passed? 88 89 @ if condition code failed to match, next insn 90 bne next @ get the next instruction; 91 92 mov r0, r6 @ prepare for EmulateAll() 93 bl EmulateAll @ emulate the instruction 94 cmp r0, #0 @ was emulation successful 95 reteq r4 @ no, return failure 96 97next: 98.Lx1: ldrt r6, [r5], #4 @ get the next instruction and 99 @ increment PC 100 101 and r2, r6, #0x0F000000 @ test for FP insns 102 teq r2, #0x0C000000 103 teqne r2, #0x0D000000 104 teqne r2, #0x0E000000 105 retne r9 @ return ok if not a fp insn 106 107 str r5, [sp, #S_PC] @ update PC copy in regs 108 109 mov r0, r6 @ save a copy 110 b emulate @ check condition and emulate 111 112 @ We need to be prepared for the instructions at .Lx1 and .Lx2 113 @ to fault. Emit the appropriate exception gunk to fix things up. 114 @ ??? For some reason, faults can happen at .Lx2 even with a 115 @ plain LDR instruction. Weird, but it seems harmless. 116 .pushsection .text.fixup,"ax" 117 .align 2 118.Lfix: ret r9 @ let the user eat segfaults 119 .popsection 120 121 .pushsection __ex_table,"a" 122 .align 3 123 .long .Lx1, .Lfix 124 .popsection 125