s390: introduce text_poke_sync()

Introduce a text_poke_sync() similar to what x86 has. This can be
used to execute a serializing instruction on all CPUs (including
the current one).

Note: according to the Principles of Operation an IPI (= interrupt)
will already serialize a CPU, however it is better to be explicit. In
addition on_each_cpu() makes sure that also the current CPU get
serialized - just to make sure that possible preemption can prevent
some theoretical case where a CPU will not be serialized.

Therefore text_poke_sync() has to be used whenever code got modified,
just to avoid to rely on implicit serialization.

Also introduce text_poke_sync_lock() which will also disable CPU
hotplug, to prevent that any CPU is just going online with a
prefetched old version of a modified instruction.

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/include/asm/text-patching.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _ASM_S390_TEXT_PATCHING_H
+#define _ASM_S390_TEXT_PATCHING_H
+
+#include <asm/barrier.h>
+
+static __always_inline void sync_core(void)
+{
+	bcr_serialize();
+}
+
+void text_poke_sync(void);
+void text_poke_sync_lock(void);
+
+#endif /* _ASM_S390_TEXT_PATCHING_H */

arch/s390/kernel/alternative.c

···
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/module.h>
+#include <linux/cpu.h>
+#include <linux/smp.h>
+#include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/facility.h>
 #include <asm/nospec-branch.h>
···
 void __init apply_alternative_instructions(void)
 {
 	apply_alternatives(__alt_instructions, __alt_instructions_end);
+}
+
+static void do_sync_core(void *info)
+{
+	sync_core();
+}
+
+void text_poke_sync(void)
+{
+	on_each_cpu(do_sync_core, NULL, 1);
+}
+
+void text_poke_sync_lock(void)
+{
+	cpus_read_lock();
+	text_poke_sync();
+	cpus_read_unlock();
 }