common-user/host/ppc: Implement safe-syscall.inc.S

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220729172141.1789105-2-richard.henderson@linaro.org>

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

@@ -0,0 +1,107 @@
+/*
+ * 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
+ *
+ * Copyright (C) 2022 Linaro, Ltd.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+/*
+ * Standardize on the _CALL_FOO symbols used by GCC:
+ * Apple XCode does not define _CALL_DARWIN.
+ * Clang defines _CALL_ELF (64-bit) but not _CALL_SYSV (32-bit).
+ */
+#if !defined(_CALL_SYSV) && \
+    !defined(_CALL_DARWIN) && \
+    !defined(_CALL_AIX) && \
+    !defined(_CALL_ELF)
+# if defined(__APPLE__)
+#  define _CALL_DARWIN
+# elif defined(__ELF__) && TCG_TARGET_REG_BITS == 32
+#  define _CALL_SYSV
+# else
+#  error "Unknown ABI"
+# endif
+#endif 
+
+#ifndef _CALL_SYSV
+# error "Unsupported ABI"
+#endif
+
+
+        .global safe_syscall_base
+        .global safe_syscall_start
+        .global safe_syscall_end
+        .type   safe_syscall_base, @function
+
+        .text
+
+        /*
+         * 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
+        stwu    1, -8(1)
+        .cfi_def_cfa_offset 8
+        stw     30, 4(1)
+        .cfi_offset 30, -4
+
+        /*
+         * We enter with r3 == &signal_pending
+         *               r4 == syscall number
+         *               r5 ... r10 == syscall arguments
+         *               and return the result in r3
+         * and the syscall instruction needs
+         *               r0 == syscall number
+         *               r3 ... r8 == syscall arguments
+         *               and returns the result in r3
+         * Shuffle everything around appropriately.
+         */
+        mr      30, 3           /* signal_pending */
+        mr      0, 4            /* syscall number */
+        mr      3, 5            /* syscall arguments */
+        mr      4, 6
+        mr      5, 7
+        mr      6, 8
+        mr      7, 9
+        mr      8, 10
+
+        /*
+         * 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 */
+        lwz     12, 0(30)
+        cmpwi   0, 12, 0
+        bne-    2f
+        sc
+safe_syscall_end:
+        /* code path when we did execute the syscall */
+        lwz     30, 4(1)        /* restore r30 */
+        addi    1, 1, 8         /* restore stack */
+        .cfi_restore 30
+        .cfi_def_cfa_offset 0
+        bnslr+                  /* return on success */
+        b       safe_syscall_set_errno_tail
+
+        /* code path when we didn't execute the syscall */
+2:      lwz     30, 4(1)
+        addi    1, 1, 8
+        addi    3, 0, QEMU_ERESTARTSYS
+        b       safe_syscall_set_errno_tail
+
+        .cfi_endproc
+
+        .size   safe_syscall_base, .-safe_syscall_base