linux-user: Provide safe_syscall for aarch64

Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
[RV] Updated syscall argument comment to match code

linux-user/host/aarch64/hostdep.h

@@ -12,4 +12,27 @@
 #ifndef QEMU_HOSTDEP_H
 #define QEMU_HOSTDEP_H
 
+/* We have a safe-syscall.inc.S */
+#define HAVE_SAFE_SYSCALL
+
+#ifndef __ASSEMBLER__
+
+/* These are defined by the safe-syscall.inc.S file */
+extern char safe_syscall_start[];
+extern char safe_syscall_end[];
+
+/* Adjust the signal context to rewind out of safe-syscall if we're in it */
+static inline void rewind_if_in_safe_syscall(void *puc)
+{
+    struct ucontext *uc = puc;
+    __u64 *pcreg = &uc->uc_mcontext.pc;
+
+    if (*pcreg > (uintptr_t)safe_syscall_start
+        && *pcreg < (uintptr_t)safe_syscall_end) {
+        *pcreg = (uintptr_t)safe_syscall_start;
+    }
+}
+
+#endif /* __ASSEMBLER__ */
+
 #endif

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

@@ -0,0 +1,75 @@
+/*
+ * 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 linux-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').
+	 * We return a long which is the syscall's return value, which
+	 * may be negative-errno on failure. Conversion to the
+	 * -1-and-errno-set convention is done by the calling wrapper.
+	 */
+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 ... x7 == 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	x6, x7
+	ldr	x7, [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, 0f 
+	svc	0x0
+safe_syscall_end:
+	/* code path for having successfully executed the syscall */
+	ret
+
+0:
+	/* code path when we didn't execute the syscall */
+	mov	x0, #-TARGET_ERESTARTSYS
+	ret
+	.cfi_endproc
+
+	.size	safe_syscall_base, .-safe_syscall_base