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>

diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index 265b13ea02eed8eb8b917b30dfb893fd460e3e08..c385a0c4057f03875d6b6326009fb68e32fdfb67 100644 (file)
--- a/arch/arm64/include/asm/alternative.h
+++ b/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__  */