alternative.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_ALTERNATIVE_H
#define _ASM_ALTERNATIVE_H

#ifndef __ASSEMBLER__

#include <linux/types.h>
#include <linux/stddef.h>
#include <linux/stringify.h>
#include <asm/asm.h>

struct alt_instr {
	s32 instr_offset;	/* offset to original instruction */
	s32 replace_offset;	/* offset to replacement instruction */
	u16 feature;		/* feature bit set for replacement */
	u8  instrlen;		/* length of original instruction */
	u8  replacementlen;	/* length of new instruction */
} __packed;

/*
 * Debug flag that can be tested to see whether alternative
 * instructions were patched in already:
 */
extern int alternatives_patched;
extern struct alt_instr __alt_instructions[], __alt_instructions_end[];

extern void alternative_instructions(void);
extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);

#define b_replacement(num)	"664"#num
#define e_replacement(num)	"665"#num

#define alt_end_marker		"663"
#define alt_slen		"662b-661b"
#define alt_total_slen		alt_end_marker"b-661b"
#define alt_rlen(num)		e_replacement(num)"f-"b_replacement(num)"f"

#define __OLDINSTR(oldinstr, num)					\
	"661:\n\t" oldinstr "\n662:\n"					\
	".fill -(((" alt_rlen(num) ")-(" alt_slen ")) > 0) * "		\
		"((" alt_rlen(num) ")-(" alt_slen ")) / 4, 4, 0x03400000\n"

#define OLDINSTR(oldinstr, num)						\
	__OLDINSTR(oldinstr, num)					\
	alt_end_marker ":\n"

#define alt_max_short(a, b)	"((" a ") ^ (((" a ") ^ (" b ")) & -(-((" a ") < (" b ")))))"

/*
 * Pad the second replacement alternative with additional NOPs if it is
 * additionally longer than the first replacement alternative.
 */
#define OLDINSTR_2(oldinstr, num1, num2) \
	"661:\n\t" oldinstr "\n662:\n"								\
	".fill -((" alt_max_short(alt_rlen(num1), alt_rlen(num2)) " - (" alt_slen ")) > 0) * "	\
		"(" alt_max_short(alt_rlen(num1), alt_rlen(num2)) " - (" alt_slen ")) / 4, "	\
		"4, 0x03400000\n"	\
	alt_end_marker ":\n"

#define ALTINSTR_ENTRY(feature, num)					      \
	" .long 661b - .\n"				/* label           */ \
	" .long " b_replacement(num)"f - .\n"		/* new instruction */ \
	" .short " __stringify(feature) "\n"		/* feature bit     */ \
	" .byte " alt_total_slen "\n"			/* source len      */ \
	" .byte " alt_rlen(num) "\n"			/* replacement len */

#define ALTINSTR_REPLACEMENT(newinstr, feature, num)	/* replacement */     \
	b_replacement(num)":\n\t" newinstr "\n" e_replacement(num) ":\n\t"

/* alternative assembly primitive: */
#define ALTERNATIVE(oldinstr, newinstr, feature)			\
	OLDINSTR(oldinstr, 1)						\
	".pushsection .altinstructions,\"a\"\n"				\
	ALTINSTR_ENTRY(feature, 1)					\
	".popsection\n"							\
	".subsection 1\n" \
	ALTINSTR_REPLACEMENT(newinstr, feature, 1)			\
	".previous\n"

#define ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2)\
	OLDINSTR_2(oldinstr, 1, 2)					\
	".pushsection .altinstructions,\"a\"\n"				\
	ALTINSTR_ENTRY(feature1, 1)					\
	ALTINSTR_ENTRY(feature2, 2)					\
	".popsection\n"							\
	".subsection 1\n" \
	ALTINSTR_REPLACEMENT(newinstr1, feature1, 1)			\
	ALTINSTR_REPLACEMENT(newinstr2, feature2, 2)			\
	".previous\n"

/*
 * Alternative instructions for different CPU types or capabilities.
 *
 * This allows to use optimized instructions even on generic binary
 * kernels.
 *
 * length of oldinstr must be longer or equal the length of newinstr
 * It can be padded with nops as needed.
 *
 * For non barrier like inlines please define new variants
 * without volatile and memory clobber.
 */
#define alternative(oldinstr, newinstr, feature)			\
	(asm volatile (ALTERNATIVE(oldinstr, newinstr, feature) : : : "memory"))

#define alternative_2(oldinstr, newinstr1, feature1, newinstr2, feature2) \
	(asm volatile(ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2) ::: "memory"))

#endif /* __ASSEMBLER__ */

#endif /* _ASM_ALTERNATIVE_H */