linux-user: Provide safe_syscall for arm

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/host/arm/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;
+    unsigned long *pcreg = &uc->uc_mcontext.arm_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/arm/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 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
+
+	.cfi_sections	.debug_frame
+
+	.text
+	.syntax unified
+	.arm
+	.align 2
+
+	/* 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:
+	.fnstart
+	.cfi_startproc
+	mov	r12, sp			/* save entry stack */
+	push	{ r4, r5, r6, r7, r8, lr }
+	.save	{ r4, r5, r6, r7, r8, lr }
+	.cfi_adjust_cfa_offset 24
+	.cfi_rel_offset r4, 0
+	.cfi_rel_offset r5, 4
+	.cfi_rel_offset r6, 8
+	.cfi_rel_offset r7, 12
+	.cfi_rel_offset r8, 16
+	.cfi_rel_offset lr, 20
+
+	/* The syscall calling convention isn't the same as the C one:
+	 * we enter with r0 == *signal_pending
+	 *               r1 == syscall number
+	 *               r2, r3, [sp+0] ... [sp+12] == syscall arguments
+	 *               and return the result in r0
+	 * and the syscall instruction needs
+	 *               r7 == syscall number
+	 *               r0 ... r6 == syscall arguments
+	 *               and returns the result in r0
+	 * Shuffle everything around appropriately.
+	 * Note the 16 bytes that we pushed to save registers.
+	 */
+	mov	r8, r0			/* copy signal_pending */
+	mov	r7, r1			/* syscall number */
+	mov	r0, r2			/* syscall args */
+	mov	r1, r3
+	ldm	r12, { r2, r3, r4, r5, r6 }
+
+	/* 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	r12, [r8]		/* signal_pending */
+	tst	r12, r12
+	bne	1f
+	swi	0
+safe_syscall_end:
+	/* code path for having successfully executed the syscall */
+	pop	{ r4, r5, r6, r7, r8, pc }
+
+1:
+	/* code path when we didn't execute the syscall */
+	ldr	r0, =-TARGET_ERESTARTSYS
+	pop	{ r4, r5, r6, r7, r8, pc }
+	.fnend
+	.cfi_endproc
+
+	.size	safe_syscall_base, .-safe_syscall_base