arm64: alternative: Work around .inst assembler bugs

AArch64 toolchains suffer from the following bug:

$ cat blah.S
1:
	.inst	0x01020304
	.if ((. - 1b) != 4)
		.error	"blah"
	.endif
$ aarch64-linux-gnu-gcc -c blah.S
blah.S: Assembler messages:
blah.S:3: Error: non-constant expression in ".if" statement

which precludes the use of msr_s and co as part of alternatives.

We workaround this issue by not directly testing the labels
themselves, but by moving the current output pointer by a value
that should always be zero. If this value is not null, then
we will trigger a backward move, which is expclicitely forbidden.
This triggers the error we're after:

  AS      arch/arm64/kvm/hyp.o
arch/arm64/kvm/hyp.S: Assembler messages:
arch/arm64/kvm/hyp.S:1377: Error: attempt to move .org backwards
scripts/Makefile.build:294: recipe for target 'arch/arm64/kvm/hyp.o' failed
make[1]: *** [arch/arm64/kvm/hyp.o] Error 1
Makefile:946: recipe for target 'arch/arm64/kvm' failed

Not pretty, but at least works on the current toolchains.
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/alternative.h

@@ -26,7 +26,20 @@ void free_alternatives_memory(void);
 	" .byte 662b-661b\n"				/* source len      */ \
 	" .byte 664f-663f\n"				/* replacement len */
 
-/* alternative assembly primitive: */
+/*
+ * alternative assembly primitive:
+ *
+ * If any of these .org directive fail, it means that insn1 and insn2
+ * don't have the same length. This used to be written as
+ *
+ * .if ((664b-663b) != (662b-661b))
+ * 	.error "Alternatives instruction length mismatch"
+ * .endif
+ *
+ * but most assemblers die if insn1 or insn2 have a .inst. This should
+ * be fixed in a binutils release posterior to 2.25.51.0.2 (anything
+ * containing commit 4e4d08cf7399b606 or c1baaddf8861).
+ */
 #define ALTERNATIVE(oldinstr, newinstr, feature)			\
 	"661:\n\t"							\
 	oldinstr "\n"							\
@@ -39,9 +52,8 @@ void free_alternatives_memory(void);
 	newinstr "\n"							\
 	"664:\n\t"							\
 	".popsection\n\t"						\
-	".if ((664b-663b) != (662b-661b))\n\t"				\
-	"	.error \"Alternatives instruction length mismatch\"\n\t"\
-	".endif\n"
+	".org	. - (664b-663b) + (662b-661b)\n\t"			\
+	".org	. - (662b-661b) + (664b-663b)\n"
 
 #else
 
@@ -61,9 +73,8 @@ void free_alternatives_memory(void);
 	.pushsection .altinstr_replacement, "ax"
 663:	\insn2
 664:	.popsection
-	.if ((664b-663b) != (662b-661b))
-		.error "Alternatives instruction length mismatch"
-	.endif
+	.org	. - (664b-663b) + (662b-661b)
+	.org	. - (662b-661b) + (664b-663b)
 .endm
 
 #endif  /*  __ASSEMBLY__  */