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/*
* 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
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