common-user/host/riscv/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) 2018 Linaro, 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 is nearly the same as C:
         * we enter with a0 == &signal_pending
         *               a1 == syscall number
         *               a2 ... a7 == syscall arguments
         *               and return the result in a0
         * and the syscall instruction needs
         *               a7 == syscall number
         *               a0 ... a5 == syscall arguments
         *               and returns the result in a0
         * Shuffle everything around appropriately.
         */
        mv      t0, a0          /* signal_pending pointer */
        mv      t1, a1          /* syscall number */
        mv      a0, a2          /* syscall arguments */
        mv      a1, a3
        mv      a2, a4
        mv      a3, a5
        mv      a4, a6
        mv      a5, a7
        mv      a7, t1

        /*
         * 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 */
        lw      t1, 0(t0)
        bnez    t1, 2f
        scall
safe_syscall_end:
        /* code path for having successfully executed the syscall */
        li      t2, -4096
        bgtu    a0, t2, 0f
        ret

        /* code path setting errno */
0:      neg     a0, a0
        j       safe_syscall_set_errno_tail

        /* code path when we didn't execute the syscall */
2:      li      a0, QEMU_ERESTARTSYS
        j       safe_syscall_set_errno_tail

        .cfi_endproc
        .size   safe_syscall_base, .-safe_syscall_base