aboutsummaryrefslogtreecommitdiff
path: root/arch/arm64/kernel/entry.S
blob: c7e047049f2c1e26db2534478b07db95ec29ede7 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
/*
 * Low-level exception handling code
 *
 * Copyright (C) 2012 ARM Ltd.
 * Authors:	Catalin Marinas <catalin.marinas@arm.com>
 *		Will Deacon <will.deacon@arm.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/init.h>
#include <linux/linkage.h>

#include <asm/assembler.h>
#include <asm/asm-offsets.h>
#include <asm/errno.h>
#include <asm/esr.h>
#include <asm/thread_info.h>
#include <asm/unistd.h>
#include <asm/unistd32.h>

/*
 * Bad Abort numbers
 *-----------------
 */
#define BAD_SYNC	0
#define BAD_IRQ		1
#define BAD_FIQ		2
#define BAD_ERROR	3

	.macro	kernel_entry, el, regsize = 64
	sub	sp, sp, #S_FRAME_SIZE - S_LR	// room for LR, SP, SPSR, ELR
	.if	\regsize == 32
	mov	w0, w0				// zero upper 32 bits of x0
	.endif
	push	x28, x29
	push	x26, x27
	push	x24, x25
	push	x22, x23
	push	x20, x21
	push	x18, x19
	push	x16, x17
	push	x14, x15
	push	x12, x13
	push	x10, x11
	push	x8, x9
	push	x6, x7
	push	x4, x5
	push	x2, x3
	push	x0, x1
	.if	\el == 0
	mrs	x21, sp_el0
	.else
	add	x21, sp, #S_FRAME_SIZE
	.endif
	mrs	x22, elr_el1
	mrs	x23, spsr_el1
	stp	lr, x21, [sp, #S_LR]
	stp	x22, x23, [sp, #S_PC]

	/*
	 * Set syscallno to -1 by default (overridden later if real syscall).
	 */
	.if	\el == 0
	mvn	x21, xzr
	str	x21, [sp, #S_SYSCALLNO]
	.endif

	/*
	 * Registers that may be useful after this macro is invoked:
	 *
	 * x21 - aborted SP
	 * x22 - aborted PC
	 * x23 - aborted PSTATE
	*/
	.endm

	.macro	kernel_exit, el, ret = 0
	ldp	x21, x22, [sp, #S_PC]		// load ELR, SPSR
	.if	\el == 0
	ldr	x23, [sp, #S_SP]		// load return stack pointer
	.endif
	.if	\ret
	ldr	x1, [sp, #S_X1]			// preserve x0 (syscall return)
	add	sp, sp, S_X2
	.else
	pop	x0, x1
	.endif
	pop	x2, x3				// load the rest of the registers
	pop	x4, x5
	pop	x6, x7
	pop	x8, x9
	msr	elr_el1, x21			// set up the return data
	msr	spsr_el1, x22
	.if	\el == 0
	msr	sp_el0, x23
	.endif
	pop	x10, x11
	pop	x12, x13
	pop	x14, x15
	pop	x16, x17
	pop	x18, x19
	pop	x20, x21
	pop	x22, x23
	pop	x24, x25
	pop	x26, x27
	pop	x28, x29
	ldr	lr, [sp], #S_FRAME_SIZE - S_LR	// load LR and restore SP
	eret					// return to kernel
	.endm

	.macro	get_thread_info, rd
	mov	\rd, sp
	and	\rd, \rd, #~((1 << 13) - 1)	// top of 8K stack
	.endm

/*
 * These are the registers used in the syscall handler, and allow us to
 * have in theory up to 7 arguments to a function - x0 to x6.
 *
 * x7 is reserved for the system call number in 32-bit mode.
 */
sc_nr	.req	x25		// number of system calls
scno	.req	x26		// syscall number
stbl	.req	x27		// syscall table pointer
tsk	.req	x28		// current thread_info

/*
 * Interrupt handling.
 */
	.macro	irq_handler
	ldr	x1, handle_arch_irq
	mov	x0, sp
	blr	x1
	.endm

	.text

/*
 * Exception vectors.
 */

	.align	11
ENTRY(vectors)
	ventry	el1_sync_invalid		// Synchronous EL1t
	ventry	el1_irq_invalid			// IRQ EL1t
	ventry	el1_fiq_invalid			// FIQ EL1t
	ventry	el1_error_invalid		// Error EL1t

	ventry	el1_sync			// Synchronous EL1h
	ventry	el1_irq				// IRQ EL1h
	ventry	el1_fiq_invalid			// FIQ EL1h
	ventry	el1_error_invalid		// Error EL1h

	ventry	el0_sync			// Synchronous 64-bit EL0
	ventry	el0_irq				// IRQ 64-bit EL0
	ventry	el0_fiq_invalid			// FIQ 64-bit EL0
	ventry	el0_error_invalid		// Error 64-bit EL0

#ifdef CONFIG_COMPAT
	ventry	el0_sync_compat			// Synchronous 32-bit EL0
	ventry	el0_irq_compat			// IRQ 32-bit EL0
	ventry	el0_fiq_invalid_compat		// FIQ 32-bit EL0
	ventry	el0_error_invalid_compat	// Error 32-bit EL0
#else
	ventry	el0_sync_invalid		// Synchronous 32-bit EL0
	ventry	el0_irq_invalid			// IRQ 32-bit EL0
	ventry	el0_fiq_invalid			// FIQ 32-bit EL0
	ventry	el0_error_invalid		// Error 32-bit EL0
#endif
END(vectors)

/*
 * Invalid mode handlers
 */
	.macro	inv_entry, el, reason, regsize = 64
	kernel_entry el, \regsize
	mov	x0, sp
	mov	x1, #\reason
	mrs	x2, esr_el1
	b	bad_mode
	.endm

el0_sync_invalid:
	inv_entry 0, BAD_SYNC
ENDPROC(el0_sync_invalid)

el0_irq_invalid:
	inv_entry 0, BAD_IRQ
ENDPROC(el0_irq_invalid)

el0_fiq_invalid:
	inv_entry 0, BAD_FIQ
ENDPROC(el0_fiq_invalid)

el0_error_invalid:
	inv_entry 0, BAD_ERROR
ENDPROC(el0_error_invalid)

#ifdef CONFIG_COMPAT
el0_fiq_invalid_compat:
	inv_entry 0, BAD_FIQ, 32
ENDPROC(el0_fiq_invalid_compat)

el0_error_invalid_compat:
	inv_entry 0, BAD_ERROR, 32
ENDPROC(el0_error_invalid_compat)
#endif

el1_sync_invalid:
	inv_entry 1, BAD_SYNC
ENDPROC(el1_sync_invalid)

el1_irq_invalid:
	inv_entry 1, BAD_IRQ
ENDPROC(el1_irq_invalid)

el1_fiq_invalid:
	inv_entry 1, BAD_FIQ
ENDPROC(el1_fiq_invalid)

el1_error_invalid:
	inv_entry 1, BAD_ERROR
ENDPROC(el1_error_invalid)

/*
 * EL1 mode handlers.
 */
	.align	6
el1_sync:
	kernel_entry 1
	mrs	x1, esr_el1			// read the syndrome register
	lsr	x24, x1, #ESR_EL1_EC_SHIFT	// exception class
	cmp	x24, #ESR_EL1_EC_DABT_EL1	// data abort in EL1
	b.eq	el1_da
	cmp	x24, #ESR_EL1_EC_SYS64		// configurable trap
	b.eq	el1_undef
	cmp	x24, #ESR_EL1_EC_SP_ALIGN	// stack alignment exception
	b.eq	el1_sp_pc
	cmp	x24, #ESR_EL1_EC_PC_ALIGN	// pc alignment exception
	b.eq	el1_sp_pc
	cmp	x24, #ESR_EL1_EC_UNKNOWN	// unknown exception in EL1
	b.eq	el1_undef
	cmp	x24, #ESR_EL1_EC_BREAKPT_EL1	// debug exception in EL1
	b.ge	el1_dbg
	b	el1_inv
el1_da:
	/*
	 * Data abort handling
	 */
	mrs	x0, far_el1
	enable_dbg_if_not_stepping x2
	// re-enable interrupts if they were enabled in the aborted context
	tbnz	x23, #7, 1f			// PSR_I_BIT
	enable_irq
1:
	mov	x2, sp				// struct pt_regs
	bl	do_mem_abort

	// disable interrupts before pulling preserved data off the stack
	disable_irq
	kernel_exit 1
el1_sp_pc:
	/*
	 * Stack or PC alignment exception handling
	 */
	mrs	x0, far_el1
	mov	x1, x25
	mov	x2, sp
	b	do_sp_pc_abort
el1_undef:
	/*
	 * Undefined instruction
	 */
	mov	x0, sp
	b	do_undefinstr
el1_dbg:
	/*
	 * Debug exception handling
	 */
	tbz	x24, #0, el1_inv		// EL1 only
	mrs	x0, far_el1
	mov	x2, sp				// struct pt_regs
	bl	do_debug_exception

	kernel_exit 1
el1_inv:
	// TODO: add support for undefined instructions in kernel mode
	mov	x0, sp
	mov	x1, #BAD_SYNC
	mrs	x2, esr_el1
	b	bad_mode
ENDPROC(el1_sync)

	.align	6
el1_irq:
	kernel_entry 1
	enable_dbg_if_not_stepping x0
#ifdef CONFIG_TRACE_IRQFLAGS
	bl	trace_hardirqs_off
#endif
#ifdef CONFIG_PREEMPT
	get_thread_info tsk
	ldr	x24, [tsk, #TI_PREEMPT]		// get preempt count
	add	x0, x24, #1			// increment it
	str	x0, [tsk, #TI_PREEMPT]
#endif
	irq_handler
#ifdef CONFIG_PREEMPT
	str	x24, [tsk, #TI_PREEMPT]		// restore preempt count
	cbnz	x24, 1f				// preempt count != 0
	ldr	x0, [tsk, #TI_FLAGS]		// get flags
	tbz	x0, #TIF_NEED_RESCHED, 1f	// needs rescheduling?
	bl	el1_preempt
1:
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
	bl	trace_hardirqs_on
#endif
	kernel_exit 1
ENDPROC(el1_irq)

#ifdef CONFIG_PREEMPT
el1_preempt:
	mov	x24, lr
1:	enable_dbg
	bl	preempt_schedule_irq		// irq en/disable is done inside
	ldr	x0, [tsk, #TI_FLAGS]		// get new tasks TI_FLAGS
	tbnz	x0, #TIF_NEED_RESCHED, 1b	// needs rescheduling?
	ret	x24
#endif

/*
 * EL0 mode handlers.
 */
	.align	6
el0_sync:
	kernel_entry 0
	mrs	x25, esr_el1			// read the syndrome register
	lsr	x24, x25, #ESR_EL1_EC_SHIFT	// exception class
	cmp	x24, #ESR_EL1_EC_SVC64		// SVC in 64-bit state
	b.eq	el0_svc
	adr	lr, ret_from_exception
	cmp	x24, #ESR_EL1_EC_DABT_EL0	// data abort in EL0
	b.eq	el0_da
	cmp	x24, #ESR_EL1_EC_IABT_EL0	// instruction abort in EL0
	b.eq	el0_ia
	cmp	x24, #ESR_EL1_EC_FP_ASIMD	// FP/ASIMD access
	b.eq	el0_fpsimd_acc
	cmp	x24, #ESR_EL1_EC_FP_EXC64	// FP/ASIMD exception
	b.eq	el0_fpsimd_exc
	cmp	x24, #ESR_EL1_EC_SYS64		// configurable trap
	b.eq	el0_undef
	cmp	x24, #ESR_EL1_EC_SP_ALIGN	// stack alignment exception
	b.eq	el0_sp_pc
	cmp	x24, #ESR_EL1_EC_PC_ALIGN	// pc alignment exception
	b.eq	el0_sp_pc
	cmp	x24, #ESR_EL1_EC_UNKNOWN	// unknown exception in EL0
	b.eq	el0_undef
	cmp	x24, #ESR_EL1_EC_BREAKPT_EL0	// debug exception in EL0
	b.ge	el0_dbg
	b	el0_inv

#ifdef CONFIG_COMPAT
	.align	6
el0_sync_compat:
	kernel_entry 0, 32
	mrs	x25, esr_el1			// read the syndrome register
	lsr	x24, x25, #ESR_EL1_EC_SHIFT	// exception class
	cmp	x24, #ESR_EL1_EC_SVC32		// SVC in 32-bit state
	b.eq	el0_svc_compat
	adr	lr, ret_from_exception
	cmp	x24, #ESR_EL1_EC_DABT_EL0	// data abort in EL0
	b.eq	el0_da
	cmp	x24, #ESR_EL1_EC_IABT_EL0	// instruction abort in EL0
	b.eq	el0_ia
	cmp	x24, #ESR_EL1_EC_FP_ASIMD	// FP/ASIMD access
	b.eq	el0_fpsimd_acc
	cmp	x24, #ESR_EL1_EC_FP_EXC32	// FP/ASIMD exception
	b.eq	el0_fpsimd_exc
	cmp	x24, #ESR_EL1_EC_UNKNOWN	// unknown exception in EL0
	b.eq	el0_undef
	cmp	x24, #ESR_EL1_EC_BREAKPT_EL0	// debug exception in EL0
	b.ge	el0_dbg
	b	el0_inv
el0_svc_compat:
	/*
	 * AArch32 syscall handling
	 */
	adr	stbl, compat_sys_call_table	// load compat syscall table pointer
	uxtw	scno, w7			// syscall number in w7 (r7)
	mov     sc_nr, #__NR_compat_syscalls
	b	el0_svc_naked

	.align	6
el0_irq_compat:
	kernel_entry 0, 32
	b	el0_irq_naked
#endif

el0_da:
	/*
	 * Data abort handling
	 */
	mrs	x0, far_el1
	disable_step x1
	isb
	enable_dbg
	// enable interrupts before calling the main handler
	enable_irq
	mov	x1, x25
	mov	x2, sp
	b	do_mem_abort
el0_ia:
	/*
	 * Instruction abort handling
	 */
	mrs	x0, far_el1
	disable_step x1
	isb
	enable_dbg
	// enable interrupts before calling the main handler
	enable_irq
	orr	x1, x25, #1 << 24		// use reserved ISS bit for instruction aborts
	mov	x2, sp
	b	do_mem_abort
el0_fpsimd_acc:
	/*
	 * Floating Point or Advanced SIMD access
	 */
	mov	x0, x25
	mov	x1, sp
	b	do_fpsimd_acc
el0_fpsimd_exc:
	/*
	 * Floating Point or Advanced SIMD exception
	 */
	mov	x0, x25
	mov	x1, sp
	b	do_fpsimd_exc
el0_sp_pc:
	/*
	 * Stack or PC alignment exception handling
	 */
	mrs	x0, far_el1
	disable_step x1
	isb
	enable_dbg
	// enable interrupts before calling the main handler
	enable_irq
	mov	x1, x25
	mov	x2, sp
	b	do_sp_pc_abort
el0_undef:
	/*
	 * Undefined instruction
	 */
	mov	x0, sp
	b	do_undefinstr
el0_dbg:
	/*
	 * Debug exception handling
	 */
	tbnz	x24, #0, el0_inv		// EL0 only
	mrs	x0, far_el1
	disable_step x1
	mov	x1, x25
	mov	x2, sp
	b	do_debug_exception
el0_inv:
	mov	x0, sp
	mov	x1, #BAD_SYNC
	mrs	x2, esr_el1
	b	bad_mode
ENDPROC(el0_sync)

	.align	6
el0_irq:
	kernel_entry 0
el0_irq_naked:
	disable_step x1
	isb
	enable_dbg
#ifdef CONFIG_TRACE_IRQFLAGS
	bl	trace_hardirqs_off
#endif
	get_thread_info tsk
#ifdef CONFIG_PREEMPT
	ldr	x24, [tsk, #TI_PREEMPT]		// get preempt count
	add	x23, x24, #1			// increment it
	str	x23, [tsk, #TI_PREEMPT]
#endif
	irq_handler
#ifdef CONFIG_PREEMPT
	ldr	x0, [tsk, #TI_PREEMPT]
	str	x24, [tsk, #TI_PREEMPT]
	cmp	x0, x23
	b.eq	1f
	mov	x1, #0
	str	x1, [x1]			// BUG
1:
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
	bl	trace_hardirqs_on
#endif
	b	ret_to_user
ENDPROC(el0_irq)

/*
 * This is the return code to user mode for abort handlers
 */
ret_from_exception:
	get_thread_info tsk
	b	ret_to_user
ENDPROC(ret_from_exception)

/*
 * Register switch for AArch64. The callee-saved registers need to be saved
 * and restored. On entry:
 *   x0 = previous task_struct (must be preserved across the switch)
 *   x1 = next task_struct
 * Previous and next are guaranteed not to be the same.
 *
 */
ENTRY(cpu_switch_to)
	add	x8, x0, #THREAD_CPU_CONTEXT
	mov	x9, sp
	stp	x19, x20, [x8], #16		// store callee-saved registers
	stp	x21, x22, [x8], #16
	stp	x23, x24, [x8], #16
	stp	x25, x26, [x8], #16
	stp	x27, x28, [x8], #16
	stp	x29, x9, [x8], #16
	str	lr, [x8]
	add	x8, x1, #THREAD_CPU_CONTEXT
	ldp	x19, x20, [x8], #16		// restore callee-saved registers
	ldp	x21, x22, [x8], #16
	ldp	x23, x24, [x8], #16
	ldp	x25, x26, [x8], #16
	ldp	x27, x28, [x8], #16
	ldp	x29, x9, [x8], #16
	ldr	lr, [x8]
	mov	sp, x9
	ret
ENDPROC(cpu_switch_to)

/*
 * This is the fast syscall return path.  We do as little as possible here,
 * and this includes saving x0 back into the kernel stack.
 */
ret_fast_syscall:
	disable_irq				// disable interrupts
	ldr	x1, [tsk, #TI_FLAGS]
	and	x2, x1, #_TIF_WORK_MASK
	cbnz	x2, fast_work_pending
	tbz	x1, #TIF_SINGLESTEP, fast_exit
	disable_dbg
	enable_step x2
fast_exit:
	kernel_exit 0, ret = 1

/*
 * Ok, we need to do extra processing, enter the slow path.
 */
fast_work_pending:
	str	x0, [sp, #S_X0]			// returned x0
work_pending:
	tbnz	x1, #TIF_NEED_RESCHED, work_resched
	/* TIF_SIGPENDING or TIF_NOTIFY_RESUME case */
	ldr	x2, [sp, #S_PSTATE]
	mov	x0, sp				// 'regs'
	tst	x2, #PSR_MODE_MASK		// user mode regs?
	b.ne	no_work_pending			// returning to kernel
	enable_irq				// enable interrupts for do_notify_resume()
	bl	do_notify_resume
	b	ret_to_user
work_resched:
	enable_dbg
	bl	schedule

/*
 * "slow" syscall return path.
 */
ret_to_user:
	disable_irq				// disable interrupts
	ldr	x1, [tsk, #TI_FLAGS]
	and	x2, x1, #_TIF_WORK_MASK
	cbnz	x2, work_pending
	tbz	x1, #TIF_SINGLESTEP, no_work_pending
	disable_dbg
	enable_step x2
no_work_pending:
	kernel_exit 0, ret = 0
ENDPROC(ret_to_user)

/*
 * This is how we return from a fork.
 */
ENTRY(ret_from_fork)
	bl	schedule_tail
	cbz	x19, 1f				// not a kernel thread
	mov	x0, x20
	blr	x19
1:	get_thread_info tsk
	b	ret_to_user
ENDPROC(ret_from_fork)

/*
 * SVC handler.
 */
	.align	6
el0_svc:
	adrp	stbl, sys_call_table		// load syscall table pointer
	uxtw	scno, w8			// syscall number in w8
	mov	sc_nr, #__NR_syscalls
el0_svc_naked:					// compat entry point
	stp	x0, scno, [sp, #S_ORIG_X0]	// save the original x0 and syscall number
	disable_step x16
	isb
	enable_dbg
	enable_irq

	get_thread_info tsk
	ldr	x16, [tsk, #TI_FLAGS]		// check for syscall tracing
	tbnz	x16, #TIF_SYSCALL_TRACE, __sys_trace // are we tracing syscalls?
	adr	lr, ret_fast_syscall		// return address
	cmp     scno, sc_nr                     // check upper syscall limit
	b.hs	ni_sys
	ldr	x16, [stbl, scno, lsl #3]	// address in the syscall table
	br	x16				// call sys_* routine
ni_sys:
	mov	x0, sp
	b	do_ni_syscall
ENDPROC(el0_svc)

	/*
	 * This is the really slow path.  We're going to be doing context
	 * switches, and waiting for our parent to respond.
	 */
__sys_trace:
	mov	x1, sp
	mov	w0, #0				// trace entry
	bl	syscall_trace
	adr	lr, __sys_trace_return		// return address
	uxtw	scno, w0			// syscall number (possibly new)
	mov	x1, sp				// pointer to regs
	cmp	scno, sc_nr			// check upper syscall limit
	b.hs	ni_sys
	ldp	x0, x1, [sp]			// restore the syscall args
	ldp	x2, x3, [sp, #S_X2]
	ldp	x4, x5, [sp, #S_X4]
	ldp	x6, x7, [sp, #S_X6]
	ldr	x16, [stbl, scno, lsl #3]	// address in the syscall table
	br	x16				// call sys_* routine

__sys_trace_return:
	str	x0, [sp]			// save returned x0
	mov	x1, sp
	mov	w0, #1				// trace exit
	bl	syscall_trace
	b	ret_to_user

/*
 * Special system call wrappers.
 */
ENTRY(sys_rt_sigreturn_wrapper)
	mov	x0, sp
	b	sys_rt_sigreturn
ENDPROC(sys_rt_sigreturn_wrapper)

ENTRY(handle_arch_irq)
	.quad	0