arm64: AArch32 user space PC alignment exception

[mirror_ubuntu-bionic-kernel.git] / arch / arm64 / kernel / entry.S

/*
 * 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/alternative.h>
#include <asm/assembler.h>
#include <asm/asm-offsets.h>
#include <asm/cpufeature.h>
#include <asm/errno.h>
#include <asm/esr.h>
#include <asm/thread_info.h>
#include <asm/unistd.h>

/*
 * Context tracking subsystem.  Used to instrument transitions
 * between user and kernel mode.
 */
        .macro ct_user_exit, syscall = 0
#ifdef CONFIG_CONTEXT_TRACKING
        bl      context_tracking_user_exit
        .if \syscall == 1
        /*
         * Save/restore needed during syscalls.  Restore syscall arguments from
         * the values already saved on stack during kernel_entry.
         */
        ldp     x0, x1, [sp]
        ldp     x2, x3, [sp, #S_X2]
        ldp     x4, x5, [sp, #S_X4]
        ldp     x6, x7, [sp, #S_X6]
        .endif
#endif
        .endm

        .macro ct_user_enter
#ifdef CONFIG_CONTEXT_TRACKING
        bl      context_tracking_user_enter
#endif
        .endm

/*
 * 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
        .if     \regsize == 32
        mov     w0, w0                          // zero upper 32 bits of x0
        .endif
        stp     x0, x1, [sp, #16 * 0]
        stp     x2, x3, [sp, #16 * 1]
        stp     x4, x5, [sp, #16 * 2]
        stp     x6, x7, [sp, #16 * 3]
        stp     x8, x9, [sp, #16 * 4]
        stp     x10, x11, [sp, #16 * 5]
        stp     x12, x13, [sp, #16 * 6]
        stp     x14, x15, [sp, #16 * 7]
        stp     x16, x17, [sp, #16 * 8]
        stp     x18, x19, [sp, #16 * 9]
        stp     x20, x21, [sp, #16 * 10]
        stp     x22, x23, [sp, #16 * 11]
        stp     x24, x25, [sp, #16 * 12]
        stp     x26, x27, [sp, #16 * 13]
        stp     x28, x29, [sp, #16 * 14]

        .if     \el == 0
        mrs     x21, sp_el0
        get_thread_info tsk                     // Ensure MDSCR_EL1.SS is clear,
        ldr     x19, [tsk, #TI_FLAGS]           // since we can unmask debug
        disable_step_tsk x19, x20               // exceptions when scheduling.
        .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
        ldp     x21, x22, [sp, #S_PC]           // load ELR, SPSR
        .if     \el == 0
        ct_user_enter
        ldr     x23, [sp, #S_SP]                // load return stack pointer
        msr     sp_el0, x23
#ifdef CONFIG_ARM64_ERRATUM_845719
alternative_if_not ARM64_WORKAROUND_845719
        nop
        nop
#ifdef CONFIG_PID_IN_CONTEXTIDR
        nop
#endif
alternative_else
        tbz     x22, #4, 1f
#ifdef CONFIG_PID_IN_CONTEXTIDR
        mrs     x29, contextidr_el1
        msr     contextidr_el1, x29
#else
        msr contextidr_el1, xzr
#endif
1:
alternative_endif
#endif
        .endif
        msr     elr_el1, x21                    // set up the return data
        msr     spsr_el1, x22
        ldp     x0, x1, [sp, #16 * 0]
        ldp     x2, x3, [sp, #16 * 1]
        ldp     x4, x5, [sp, #16 * 2]
        ldp     x6, x7, [sp, #16 * 3]
        ldp     x8, x9, [sp, #16 * 4]
        ldp     x10, x11, [sp, #16 * 5]
        ldp     x12, x13, [sp, #16 * 6]
        ldp     x14, x15, [sp, #16 * 7]
        ldp     x16, x17, [sp, #16 * 8]
        ldp     x18, x19, [sp, #16 * 9]
        ldp     x20, x21, [sp, #16 * 10]
        ldp     x22, x23, [sp, #16 * 11]
        ldp     x24, x25, [sp, #16 * 12]
        ldp     x26, x27, [sp, #16 * 13]
        ldp     x28, x29, [sp, #16 * 14]
        ldr     lr, [sp, #S_LR]
        add     sp, sp, #S_FRAME_SIZE           // restore sp
        eret                                    // return to kernel
        .endm

        .macro  get_thread_info, rd
        mov     \rd, sp
        and     \rd, \rd, #~(THREAD_SIZE - 1)   // top of 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
        adrp    x1, handle_arch_irq
        ldr     x1, [x1, #:lo12: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_ELx_EC_SHIFT      // exception class
        cmp     x24, #ESR_ELx_EC_DABT_CUR       // data abort in EL1
        b.eq    el1_da
        cmp     x24, #ESR_ELx_EC_SYS64          // configurable trap
        b.eq    el1_undef
        cmp     x24, #ESR_ELx_EC_SP_ALIGN       // stack alignment exception
        b.eq    el1_sp_pc
        cmp     x24, #ESR_ELx_EC_PC_ALIGN       // pc alignment exception
        b.eq    el1_sp_pc
        cmp     x24, #ESR_ELx_EC_UNKNOWN        // unknown exception in EL1
        b.eq    el1_undef
        cmp     x24, #ESR_ELx_EC_BREAKPT_CUR    // debug exception in EL1
        b.ge    el1_dbg
        b       el1_inv
el1_da:
        /*
         * Data abort handling
         */
        mrs     x0, far_el1
        enable_dbg
        // 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
        enable_dbg
        mov     x2, sp
        b       do_sp_pc_abort
el1_undef:
        /*
         * Undefined instruction
         */
        enable_dbg
        mov     x0, sp
        b       do_undefinstr
el1_dbg:
        /*
         * Debug exception handling
         */
        cmp     x24, #ESR_ELx_EC_BRK64          // if BRK64
        cinc    x24, x24, eq                    // set bit '0'
        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
        enable_dbg
        mov     x0, sp
        mov     x2, x1
        mov     x1, #BAD_SYNC
        b       bad_mode
ENDPROC(el1_sync)

        .align  6
el1_irq:
        kernel_entry 1
        enable_dbg
#ifdef CONFIG_TRACE_IRQFLAGS
        bl      trace_hardirqs_off
#endif

        irq_handler

#ifdef CONFIG_PREEMPT
        get_thread_info tsk
        ldr     w24, [tsk, #TI_PREEMPT]         // get preempt count
        cbnz    w24, 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:      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_ELx_EC_SHIFT     // exception class
        cmp     x24, #ESR_ELx_EC_SVC64          // SVC in 64-bit state
        b.eq    el0_svc
        cmp     x24, #ESR_ELx_EC_DABT_LOW       // data abort in EL0
        b.eq    el0_da
        cmp     x24, #ESR_ELx_EC_IABT_LOW       // instruction abort in EL0
        b.eq    el0_ia
        cmp     x24, #ESR_ELx_EC_FP_ASIMD       // FP/ASIMD access
        b.eq    el0_fpsimd_acc
        cmp     x24, #ESR_ELx_EC_FP_EXC64       // FP/ASIMD exception
        b.eq    el0_fpsimd_exc
        cmp     x24, #ESR_ELx_EC_SYS64          // configurable trap
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_SP_ALIGN       // stack alignment exception
        b.eq    el0_sp_pc
        cmp     x24, #ESR_ELx_EC_PC_ALIGN       // pc alignment exception
        b.eq    el0_sp_pc
        cmp     x24, #ESR_ELx_EC_UNKNOWN        // unknown exception in EL0
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_BREAKPT_LOW    // 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_ELx_EC_SHIFT     // exception class
        cmp     x24, #ESR_ELx_EC_SVC32          // SVC in 32-bit state
        b.eq    el0_svc_compat
        cmp     x24, #ESR_ELx_EC_DABT_LOW       // data abort in EL0
        b.eq    el0_da
        cmp     x24, #ESR_ELx_EC_IABT_LOW       // instruction abort in EL0
        b.eq    el0_ia
        cmp     x24, #ESR_ELx_EC_FP_ASIMD       // FP/ASIMD access
        b.eq    el0_fpsimd_acc
        cmp     x24, #ESR_ELx_EC_FP_EXC32       // FP/ASIMD exception
        b.eq    el0_fpsimd_exc
        cmp     x24, #ESR_ELx_EC_PC_ALIGN       // pc alignment exception
        b.eq    el0_sp_pc
        cmp     x24, #ESR_ELx_EC_UNKNOWN        // unknown exception in EL0
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_CP15_32        // CP15 MRC/MCR trap
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_CP15_64        // CP15 MRRC/MCRR trap
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_CP14_MR        // CP14 MRC/MCR trap
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_CP14_LS        // CP14 LDC/STC trap
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_CP14_64        // CP14 MRRC/MCRR trap
        b.eq    el0_undef
        cmp     x24, #ESR_ELx_EC_BREAKPT_LOW    // debug exception in EL0
        b.ge    el0_dbg
        b       el0_inv
el0_svc_compat:
        /*
         * AArch32 syscall handling
         */
        adrp    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     x26, far_el1
        // enable interrupts before calling the main handler
        enable_dbg_and_irq
        ct_user_exit
        bic     x0, x26, #(0xff << 56)
        mov     x1, x25
        mov     x2, sp
        bl      do_mem_abort
        b       ret_to_user
el0_ia:
        /*
         * Instruction abort handling
         */
        mrs     x26, far_el1
        // enable interrupts before calling the main handler
        enable_dbg_and_irq
        ct_user_exit
        mov     x0, x26
        orr     x1, x25, #1 << 24               // use reserved ISS bit for instruction aborts
        mov     x2, sp
        bl      do_mem_abort
        b       ret_to_user
el0_fpsimd_acc:
        /*
         * Floating Point or Advanced SIMD access
         */
        enable_dbg
        ct_user_exit
        mov     x0, x25
        mov     x1, sp
        bl      do_fpsimd_acc
        b       ret_to_user
el0_fpsimd_exc:
        /*
         * Floating Point or Advanced SIMD exception
         */
        enable_dbg
        ct_user_exit
        mov     x0, x25
        mov     x1, sp
        bl      do_fpsimd_exc
        b       ret_to_user
el0_sp_pc:
        /*
         * Stack or PC alignment exception handling
         */
        mrs     x26, far_el1
        // enable interrupts before calling the main handler
        enable_dbg_and_irq
        ct_user_exit
        mov     x0, x26
        mov     x1, x25
        mov     x2, sp
        bl      do_sp_pc_abort
        b       ret_to_user
el0_undef:
        /*
         * Undefined instruction
         */
        // enable interrupts before calling the main handler
        enable_dbg_and_irq
        ct_user_exit
        mov     x0, sp
        bl      do_undefinstr
        b       ret_to_user
el0_dbg:
        /*
         * Debug exception handling
         */
        tbnz    x24, #0, el0_inv                // EL0 only
        mrs     x0, far_el1
        mov     x1, x25
        mov     x2, sp
        bl      do_debug_exception
        enable_dbg
        ct_user_exit
        b       ret_to_user
el0_inv:
        enable_dbg
        ct_user_exit
        mov     x0, sp
        mov     x1, #BAD_SYNC
        mov     x2, x25
        bl      bad_mode
        b       ret_to_user
ENDPROC(el0_sync)

        .align  6
el0_irq:
        kernel_entry 0
el0_irq_naked:
        enable_dbg
#ifdef CONFIG_TRACE_IRQFLAGS
        bl      trace_hardirqs_off
#endif

        ct_user_exit
        irq_handler

#ifdef CONFIG_TRACE_IRQFLAGS
        bl      trace_hardirqs_on
#endif
        b       ret_to_user
ENDPROC(el0_irq)

/*
 * 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)
        mov     x10, #THREAD_CPU_CONTEXT
        add     x8, x0, x10
        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, x10
        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
        str     x0, [sp, #S_X0]                 // returned x0
        ldr     x1, [tsk, #TI_FLAGS]            // re-check for syscall tracing
        and     x2, x1, #_TIF_SYSCALL_WORK
        cbnz    x2, ret_fast_syscall_trace
        and     x2, x1, #_TIF_WORK_MASK
        cbnz    x2, work_pending
        enable_step_tsk x1, x2
        kernel_exit 0
ret_fast_syscall_trace:
        enable_irq                              // enable interrupts
        b       __sys_trace_return_skipped      // we already saved x0

/*
 * Ok, we need to do extra processing, enter the slow path.
 */
work_pending:
        tbnz    x1, #TIF_NEED_RESCHED, work_resched
        /* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE 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:
        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
        enable_step_tsk x1, x2
no_work_pending:
        kernel_exit 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
        enable_dbg_and_irq
        ct_user_exit 1

        ldr     x16, [tsk, #TI_FLAGS]           // check for syscall hooks
        tst     x16, #_TIF_SYSCALL_WORK
        b.ne    __sys_trace
        cmp     scno, sc_nr                     // check upper syscall limit
        b.hs    ni_sys
        ldr     x16, [stbl, scno, lsl #3]       // address in the syscall table
        blr     x16                             // call sys_* routine
        b       ret_fast_syscall
ni_sys:
        mov     x0, sp
        bl      do_ni_syscall
        b       ret_fast_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     w0, #-1                         // set default errno for
        cmp     scno, x0                        // user-issued syscall(-1)
        b.ne    1f
        mov     x0, #-ENOSYS
        str     x0, [sp, #S_X0]
1:      mov     x0, sp
        bl      syscall_trace_enter
        cmp     w0, #-1                         // skip the syscall?
        b.eq    __sys_trace_return_skipped
        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_trace
        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
        blr     x16                             // call sys_* routine

__sys_trace_return:
        str     x0, [sp, #S_X0]                 // save returned x0
__sys_trace_return_skipped:
        mov     x0, sp
        bl      syscall_trace_exit
        b       ret_to_user

__ni_sys_trace:
        mov     x0, sp
        bl      do_ni_syscall
        b       __sys_trace_return

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