s390/exceptions: switch to relative exception table entries

This is the s390 port of 70627654 "x86, extable: Switch to relative
exception table entries".
Reduces the size of our exception tables by 50% on 64 bit builds.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/extable.c

@@ -0,0 +1,81 @@
+#include <linux/module.h>
+#include <linux/sort.h>
+#include <asm/uaccess.h>
+
+/*
+ * Search one exception table for an entry corresponding to the
+ * given instruction address, and return the address of the entry,
+ * or NULL if none is found.
+ * We use a binary search, and thus we assume that the table is
+ * already sorted.
+ */
+const struct exception_table_entry *
+search_extable(const struct exception_table_entry *first,
+	       const struct exception_table_entry *last,
+	       unsigned long value)
+{
+	const struct exception_table_entry *mid;
+	unsigned long addr;
+
+	while (first <= last) {
+		mid = ((last - first) >> 1) + first;
+		addr = extable_insn(mid);
+		if (addr < value)
+			first = mid + 1;
+		else if (addr > value)
+			last = mid - 1;
+		else
+			return mid;
+	}
+	return NULL;
+}
+
+/*
+ * The exception table needs to be sorted so that the binary
+ * search that we use to find entries in it works properly.
+ * This is used both for the kernel exception table and for
+ * the exception tables of modules that get loaded.
+ *
+ */
+static int cmp_ex(const void *a, const void *b)
+{
+	const struct exception_table_entry *x = a, *y = b;
+
+	/* This compare is only valid after normalization. */
+	return x->insn - y->insn;
+}
+
+void sort_extable(struct exception_table_entry *start,
+		  struct exception_table_entry *finish)
+{
+	struct exception_table_entry *p;
+	int i;
+
+	/* Normalize entries to being relative to the start of the section */
+	for (p = start, i = 0; p < finish; p++, i += 8)
+		p->insn += i;
+	sort(start, finish - start, sizeof(*start), cmp_ex, NULL);
+	/* Denormalize all entries */
+	for (p = start, i = 0; p < finish; p++, i += 8)
+		p->insn -= i;
+}
+
+#ifdef CONFIG_MODULES
+/*
+ * If the exception table is sorted, any referring to the module init
+ * will be at the beginning or the end.
+ */
+void trim_init_extable(struct module *m)
+{
+	/* Trim the beginning */
+	while (m->num_exentries &&
+	       within_module_init(extable_insn(&m->extable[0]), m)) {
+		m->extable++;
+		m->num_exentries--;
+	}
+	/* Trim the end */
+	while (m->num_exentries &&
+	       within_module_init(extable_insn(&m->extable[m->num_exentries-1]), m))
+		m->num_exentries--;
+}
+#endif /* CONFIG_MODULES */