1/*
2 * safe-syscall.inc.S : host-specific assembly fragment
3 * to handle signals occurring at the same time as system calls.
4 * This is intended to be included by common-user/safe-syscall.S
5 *
6 * Written by Richard Henderson <rth@twiddle.net>
7 * Copyright (C) 2018 Linaro, Inc.
8 *
9 * This work is licensed under the terms of the GNU GPL, version 2 or later.
10 * See the COPYING file in the top-level directory.
11 */
12
13        .global safe_syscall_base
14        .global safe_syscall_start
15        .global safe_syscall_end
16        .type   safe_syscall_base, @function
17        .type   safe_syscall_start, @function
18        .type   safe_syscall_end, @function
19
20        /*
21         * This is the entry point for making a system call. The calling
22         * convention here is that of a C varargs function with the
23         * first argument an 'int *' to the signal_pending flag, the
24         * second one the system call number (as a 'long'), and all further
25         * arguments being syscall arguments (also 'long').
26         */
27safe_syscall_base:
28        .cfi_startproc
29        /*
30         * The syscall calling convention is nearly the same as C:
31         * we enter with a0 == &signal_pending
32         *               a1 == syscall number
33         *               a2 ... a7 == syscall arguments
34         *               and return the result in a0
35         * and the syscall instruction needs
36         *               a7 == syscall number
37         *               a0 ... a5 == syscall arguments
38         *               and returns the result in a0
39         * Shuffle everything around appropriately.
40         */
41        mv      t0, a0          /* signal_pending pointer */
42        mv      t1, a1          /* syscall number */
43        mv      a0, a2          /* syscall arguments */
44        mv      a1, a3
45        mv      a2, a4
46        mv      a3, a5
47        mv      a4, a6
48        mv      a5, a7
49        mv      a7, t1
50
51        /*
52         * This next sequence of code works in conjunction with the
53         * rewind_if_safe_syscall_function(). If a signal is taken
54         * and the interrupted PC is anywhere between 'safe_syscall_start'
55         * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
56         * The code sequence must therefore be able to cope with this, and
57         * the syscall instruction must be the final one in the sequence.
58         */
59safe_syscall_start:
60        /* If signal_pending is non-zero, don't do the call */
61        lw      t1, 0(t0)
62        bnez    t1, 2f
63        scall
64safe_syscall_end:
65        /* code path for having successfully executed the syscall */
66        li      t2, -4096
67        bgtu    a0, t2, 0f
68        ret
69
70        /* code path setting errno */
710:      neg     a0, a0
72        j       safe_syscall_set_errno_tail
73
74        /* code path when we didn't execute the syscall */
752:      li      a0, QEMU_ERESTARTSYS
76        j       safe_syscall_set_errno_tail
77
78        .cfi_endproc
79        .size   safe_syscall_base, .-safe_syscall_base
80