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