common-user: Move safe-syscall.* from linux-user

Move linux-user safe-syscall.S and safe-syscall-error.c to common-user
so that bsd-user can also use it.  Also move safe-syscall.h to
include/user/.  Since there is nothing here that is related to the guest,
as opposed to the host, build it once.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

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

@@ -0,0 +1,76 @@
+/*
+ * 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 */
+        cmp     x0, #-4096
+        b.hi    0f
+        ret
+
+        /* code path setting errno */
+0:      neg     w0, w0
+        b       safe_syscall_set_errno_tail
+
+        /* code path when we didn't execute the syscall */
+2:      mov     w0, #QEMU_ERESTARTSYS
+        b       safe_syscall_set_errno_tail
+        .cfi_endproc
+        .size   safe_syscall_base, .-safe_syscall_base