common-user/host/aarch64/safe-syscall.inc.S

/*
 * safe-syscall.inc.S : host-specific assembly fragment
 * to handle signals occurring at the same time as system calls.
 * This is intended to be included by common-user/safe-syscall.S
 *
 * Written by Richard Henderson <rth@twiddle.net>
 * Copyright (C) 2016 Red Hat, Inc.
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or later.
 * See the COPYING file in the top-level directory.
 */

        .global safe_syscall_base
        .global safe_syscall_start
        .global safe_syscall_end
        .type   safe_syscall_base, #function
        .type   safe_syscall_start, #function
        .type   safe_syscall_end, #function

        /* This is the entry point for making a system call. The calling
         * convention here is that of a C varargs function with the
         * first argument an 'int *' to the signal_pending flag, the
         * second one the system call number (as a 'long'), and all further
         * arguments being syscall arguments (also 'long').
         */
safe_syscall_base:
        .cfi_startproc
        /* The syscall calling convention isn't the same as the
         * C one:
         * we enter with x0 == &signal_pending
         *               x1 == syscall number
         *               x2 ... x7, (stack) == syscall arguments
         *               and return the result in x0
         * and the syscall instruction needs
         *               x8 == syscall number
         *               x0 ... x6 == syscall arguments
         *               and returns the result in x0
         * Shuffle everything around appropriately.
         */
        mov     x9, x0          /* signal_pending pointer */
        mov     x8, x1          /* syscall number */
        mov     x0, x2          /* syscall arguments */
        mov     x1, x3
        mov     x2, x4
        mov     x3, x5
        mov     x4, x6
        mov     x5, x7
        ldr     x6, [sp]

        /* This next sequence of code works in conjunction with the
         * rewind_if_safe_syscall_function(). If a signal is taken
         * and the interrupted PC is anywhere between 'safe_syscall_start'
         * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
         * The code sequence must therefore be able to cope with this, and
         * the syscall instruction must be the final one in the sequence.
         */
safe_syscall_start:
        /* if signal_pending is non-zero, don't do the call */
        ldr     w10, [x9]
        cbnz    w10, 2f
        svc     0x0
safe_syscall_end:

        /* code path for having successfully executed the syscall */
#if defined(__linux__)
        /* Linux kernel returns (small) negative errno. */
        cmp     x0, #-4096
        b.hi    0f
#elif defined(__FreeBSD__)
        /* FreeBSD kernel returns positive errno and C bit set. */
        b.cs    1f
#else
#error "unsupported os"
#endif
        ret

#if defined(__linux__)
        /* code path setting errno */
0:      neg     w0, w0
        b       safe_syscall_set_errno_tail
#endif

        /* code path when we didn't execute the syscall */
2:      mov     w0, #QEMU_ERESTARTSYS
1:      b       safe_syscall_set_errno_tail

        .cfi_endproc
        .size   safe_syscall_base, .-safe_syscall_base