common-user: Add safe syscall handling for loongarch64 hosts

Signed-off-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211221054105.178795-29-git@xen0n.name>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

common-user/host/loongarch64/safe-syscall.inc.S

@@ -0,0 +1,90 @@
+/*
+ * 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
+ *
+ * Ported to LoongArch by WANG Xuerui <git@xen0n.name>
+ *
+ * Based on safe-syscall.inc.S code for RISC-V,
+ * originally 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.
+         */
+        move    $t0, $a0        /* signal_pending pointer */
+        move    $t1, $a1        /* syscall number */
+        move    $a0, $a2        /* syscall arguments */
+        move    $a1, $a3
+        move    $a2, $a4
+        move    $a3, $a5
+        move    $a4, $a6
+        move    $a5, $a7
+        move    $a7, $t1
+
+        /*
+         * We need to preserve the signal_pending pointer but t0 is
+         * clobbered by syscalls on LoongArch, so we need to move it
+         * somewhere else, ideally both preserved across syscalls and
+         * clobbered by procedure calls so we don't have to allocate a
+         * stack frame; a6 is just the register we want here.
+         */
+        move    $a6, $t0
+
+        /*
+         * 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 */
+        ld.w    $t1, $a6, 0
+        bnez    $t1, 2f
+        syscall 0
+safe_syscall_end:
+        /* code path for having successfully executed the syscall */
+        li.w    $t2, -4096
+        bgtu    $a0, $t2, 0f
+        jr      $ra
+
+        /* code path setting errno */
+0:      sub.d   $a0, $zero, $a0
+        b       safe_syscall_set_errno_tail
+
+        /* code path when we didn't execute the syscall */
+2:      li.w    $a0, QEMU_ERESTARTSYS
+        b       safe_syscall_set_errno_tail
+        .cfi_endproc
+        .size   safe_syscall_base, .-safe_syscall_base