s390: add assembler macros for CPU alternatives

Add a header with macros usable in assembler files to emit alternative
code sequences. It works analog to the alternatives for inline assmeblies
in C files, with the same restrictions and capabilities.
The syntax is

     ALTERNATIVE "<default instructions sequence>", \
		 "<alternative instructions sequence>", \
		 "<features-bit>"
and

     ALTERNATIVE_2 "<default instructions sequence>", \
		   "<alternative instructions sqeuence #1>", \
		   "<feature-bit #1>",
		   "<alternative instructions sqeuence #2>", \
		   "<feature-bit #2>"
Reviewed-by: NVasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/alternative-asm.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_S390_ALTERNATIVE_ASM_H
+#define _ASM_S390_ALTERNATIVE_ASM_H
+
+#ifdef __ASSEMBLY__
+
+/*
+ * Check the length of an instruction sequence. The length may not be larger
+ * than 254 bytes and it has to be divisible by 2.
+ */
+.macro alt_len_check start,end
+	.if ( \end - \start ) > 254
+	.error "cpu alternatives does not support instructions blocks > 254 bytes\n"
+	.endif
+	.if ( \end - \start ) % 2
+	.error "cpu alternatives instructions length is odd\n"
+	.endif
+.endm
+
+/*
+ * Issue one struct alt_instr descriptor entry (need to put it into
+ * the section .altinstructions, see below). This entry contains
+ * enough information for the alternatives patching code to patch an
+ * instruction. See apply_alternatives().
+ */
+.macro alt_entry orig_start, orig_end, alt_start, alt_end, feature
+	.long	\orig_start - .
+	.long	\alt_start - .
+	.word	\feature
+	.byte	\orig_end - \orig_start
+	.byte	\alt_end - \alt_start
+.endm
+
+/*
+ * Fill up @bytes with nops. The macro emits 6-byte nop instructions
+ * for the bulk of the area, possibly followed by a 4-byte and/or
+ * a 2-byte nop if the size of the area is not divisible by 6.
+ */
+.macro alt_pad_fill bytes
+	.fill	( \bytes ) / 6, 6, 0xc0040000
+	.fill	( \bytes ) % 6 / 4, 4, 0x47000000
+	.fill	( \bytes ) % 6 % 4 / 2, 2, 0x0700
+.endm
+
+/*
+ * Fill up @bytes with nops. If the number of bytes is larger
+ * than 6, emit a jg instruction to branch over all nops, then
+ * fill an area of size (@bytes - 6) with nop instructions.
+ */
+.macro alt_pad bytes
+	.if ( \bytes > 0 )
+	.if ( \bytes > 6 )
+	jg	. + \bytes
+	alt_pad_fill \bytes - 6
+	.else
+	alt_pad_fill \bytes
+	.endif
+	.endif
+.endm
+
+/*
+ * Define an alternative between two instructions. If @feature is
+ * present, early code in apply_alternatives() replaces @oldinstr with
+ * @newinstr. ".skip" directive takes care of proper instruction padding
+ * in case @newinstr is longer than @oldinstr.
+ */
+.macro ALTERNATIVE oldinstr, newinstr, feature
+	.pushsection .altinstr_replacement,"ax"
+770:	\newinstr
+771:	.popsection
+772:	\oldinstr
+773:	alt_len_check 770b, 771b
+	alt_len_check 772b, 773b
+	alt_pad ( ( 771b - 770b ) - ( 773b - 772b ) )
+774:	.pushsection .altinstructions,"a"
+	alt_entry 772b, 774b, 770b, 771b, \feature
+	.popsection
+.endm
+
+/*
+ * Define an alternative between two instructions. If @feature is
+ * present, early code in apply_alternatives() replaces @oldinstr with
+ * @newinstr. ".skip" directive takes care of proper instruction padding
+ * in case @newinstr is longer than @oldinstr.
+ */
+.macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2
+	.pushsection .altinstr_replacement,"ax"
+770:	\newinstr1
+771:	\newinstr2
+772:	.popsection
+773:	\oldinstr
+774:	alt_len_check 770b, 771b
+	alt_len_check 771b, 772b
+	alt_len_check 773b, 774b
+	.if ( 771b - 770b > 772b - 771b )
+	alt_pad ( ( 771b - 770b ) - ( 774b - 773b ) )
+	.else
+	alt_pad ( ( 772b - 771b ) - ( 774b - 773b ) )
+	.endif
+775:	.pushsection .altinstructions,"a"
+	alt_entry 773b, 775b, 770b, 771b,\feature1
+	alt_entry 773b, 775b, 771b, 772b,\feature2
+	.popsection
+.endm
+
+#endif	/*  __ASSEMBLY__  */
+
+#endif /* _ASM_S390_ALTERNATIVE_ASM_H */