s390: introduce CPU alternatives

Implement CPU alternatives, which allows to optionally patch newer
instructions at runtime, based on CPU facilities availability.

A new kernel boot parameter "noaltinstr" disables patching.

Current implementation is derived from x86 alternatives. Although
ideal instructions padding (when altinstr is longer then oldinstr)
is added at compile time, and no oldinstr nops optimization has to be
done at runtime. Also couple of compile time sanity checks are done:
1. oldinstr and altinstr must be <= 254 bytes long,
2. oldinstr and altinstr must not have an odd length.

alternative(oldinstr, altinstr, facility);
alternative_2(oldinstr, altinstr1, facility1, altinstr2, facility2);

Both compile time and runtime padding consists of either 6/4/2 bytes nop
or a jump (brcl) + 2 bytes nop filler if padding is longer then 6 bytes.

.altinstructions and .altinstr_replacement sections are part of
__init_begin : __init_end region and are freed after initialization.

Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

5 files changed, 154 insertions, 0 deletions

diff --git a/arch/s390/kernel/Makefile b/arch/s390/kernel/Makefile
index 7b742ffac7d3..addf61024153 100644
--- a/arch/s390/kernel/Makefile
+++ b/arch/s390/kernel/Makefile
@@ -76,6 +76,7 @@ obj-$(CONFIG_KPROBES)		+= kprobes.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o ftrace.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
 obj-$(CONFIG_UPROBES)		+= uprobes.o
+obj-$(CONFIG_ALTERNATIVES)	+= alternative.o
 
 obj-$(CONFIG_PERF_EVENTS)	+= perf_event.o perf_cpum_cf.o perf_cpum_sf.o
 obj-$(CONFIG_PERF_EVENTS)	+= perf_cpum_cf_events.o
diff --git a/arch/s390/kernel/alternative.c b/arch/s390/kernel/alternative.c
new file mode 100644
index 000000000000..315986a06cf5
--- /dev/null
+++ b/arch/s390/kernel/alternative.c
@@ -0,0 +1,110 @@
+#include <linux/module.h>
+#include <asm/alternative.h>
+#include <asm/facility.h>
+
+#define MAX_PATCH_LEN (255 - 1)
+
+static int __initdata_or_module alt_instr_disabled;
+
+static int __init disable_alternative_instructions(char *str)
+{
+	alt_instr_disabled = 1;
+	return 0;
+}
+
+early_param("noaltinstr", disable_alternative_instructions);
+
+struct brcl_insn {
+	u16 opc;
+	s32 disp;
+} __packed;
+
+static u16 __initdata_or_module nop16 = 0x0700;
+static u32 __initdata_or_module nop32 = 0x47000000;
+static struct brcl_insn __initdata_or_module nop48 = {
+	0xc004, 0
+};
+
+static const void *nops[] __initdata_or_module = {
+	&nop16,
+	&nop32,
+	&nop48
+};
+
+static void __init_or_module add_jump_padding(void *insns, unsigned int len)
+{
+	struct brcl_insn brcl = {
+		0xc0f4,
+		len / 2
+	};
+
+	memcpy(insns, &brcl, sizeof(brcl));
+	insns += sizeof(brcl);
+	len -= sizeof(brcl);
+
+	while (len > 0) {
+		memcpy(insns, &nop16, 2);
+		insns += 2;
+		len -= 2;
+	}
+}
+
+static void __init_or_module add_padding(void *insns, unsigned int len)
+{
+	if (len > 6)
+		add_jump_padding(insns, len);
+	else if (len >= 2)
+		memcpy(insns, nops[len / 2 - 1], len);
+}
+
+static void __init_or_module __apply_alternatives(struct alt_instr *start,
+						  struct alt_instr *end)
+{
+	struct alt_instr *a;
+	u8 *instr, *replacement;
+	u8 insnbuf[MAX_PATCH_LEN];
+
+	/*
+	 * The scan order should be from start to end. A later scanned
+	 * alternative code can overwrite previously scanned alternative code.
+	 */
+	for (a = start; a < end; a++) {
+		int insnbuf_sz = 0;
+
+		instr = (u8 *)&a->instr_offset + a->instr_offset;
+		replacement = (u8 *)&a->repl_offset + a->repl_offset;
+
+		if (!test_facility(a->facility))
+			continue;
+
+		if (unlikely(a->instrlen % 2 || a->replacementlen % 2)) {
+			WARN_ONCE(1, "cpu alternatives instructions length is "
+				     "odd, skipping patching\n");
+			continue;
+		}
+
+		memcpy(insnbuf, replacement, a->replacementlen);
+		insnbuf_sz = a->replacementlen;
+
+		if (a->instrlen > a->replacementlen) {
+			add_padding(insnbuf + a->replacementlen,
+				    a->instrlen - a->replacementlen);
+			insnbuf_sz += a->instrlen - a->replacementlen;
+		}
+
+		s390_kernel_write(instr, insnbuf, insnbuf_sz);
+	}
+}
+
+void __init_or_module apply_alternatives(struct alt_instr *start,
+					 struct alt_instr *end)
+{
+	if (!alt_instr_disabled)
+		__apply_alternatives(start, end);
+}
+
+extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
+void __init apply_alternative_instructions(void)
+{
+	apply_alternatives(__alt_instructions, __alt_instructions_end);
+}
diff --git a/arch/s390/kernel/module.c b/arch/s390/kernel/module.c
index 1a27f307a920..6d9f73bb4142 100644
--- a/arch/s390/kernel/module.c
+++ b/arch/s390/kernel/module.c
@@ -31,6 +31,7 @@
 #include <linux/kernel.h>
 #include <linux/moduleloader.h>
 #include <linux/bug.h>
+#include <asm/alternative.h>
 
 #if 0
 #define DEBUGP printk
@@ -429,6 +430,22 @@ int module_finalize(const Elf_Ehdr *hdr,
 		    const Elf_Shdr *sechdrs,
 		    struct module *me)
 {
+	const Elf_Shdr *s;
+	char *secstrings;
+
+	if (IS_ENABLED(CONFIG_ALTERNATIVES)) {
+		secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
+		for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
+			if (!strcmp(".altinstructions",
+				    secstrings + s->sh_name)) {
+				/* patch .altinstructions */
+				void *aseg = (void *)s->sh_addr;
+
+				apply_alternatives(aseg, aseg + s->sh_size);
+			}
+		}
+	}
+
 	jump_label_apply_nops(me);
 	return 0;
 }
diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
index b2c9af9b88d5..c07e6d6a91cc 100644
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -66,6 +66,7 @@
 #include <asm/sclp.h>
 #include <asm/sysinfo.h>
 #include <asm/numa.h>
+#include <asm/alternative.h>
 #include "entry.h"
 
 /*
@@ -957,6 +958,8 @@ void __init setup_arch(char **cmdline_p)
 	conmode_default();
 	set_preferred_console();
 
+	apply_alternative_instructions();
+
 	/* Setup zfcpdump support */
 	setup_zfcpdump();
 
diff --git a/arch/s390/kernel/vmlinux.lds.S b/arch/s390/kernel/vmlinux.lds.S
index 6e2c42bd1c3b..7c9fcf7cb43d 100644
--- a/arch/s390/kernel/vmlinux.lds.S
+++ b/arch/s390/kernel/vmlinux.lds.S
@@ -104,6 +104,29 @@ SECTIONS
 		EXIT_DATA
 	}
 
+	/*
+	 * struct alt_inst entries. From the header (alternative.h):
+	 * "Alternative instructions for different CPU types or capabilities"
+	 * Think locking instructions on spinlocks.
+	 * Note, that it is a part of __init region.
+	 */
+	. = ALIGN(8);
+	.altinstructions : {
+		__alt_instructions = .;
+		*(.altinstructions)
+		__alt_instructions_end = .;
+	}
+
+	/*
+	 * And here are the replacement instructions. The linker sticks
+	 * them as binary blobs. The .altinstructions has enough data to
+	 * get the address and the length of them to patch the kernel safely.
+	 * Note, that it is a part of __init region.
+	 */
+	.altinstr_replacement : {
+		*(.altinstr_replacement)
+	}
+
 	/* early.c uses stsi, which requires page aligned data. */
 	. = ALIGN(PAGE_SIZE);
 	INIT_DATA_SECTION(0x100)