Commit eb7c11ee authored by Marc Zyngier's avatar Marc Zyngier Committed by Catalin Marinas

arm64: alternative: Work around .inst assembler bugs

AArch64 toolchains suffer from the following bug:

$ cat blah.S
	.inst	0x01020304
	.if ((. - 1b) != 4)
		.error	"blah"
$ 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/ 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: default avatarWill Deacon <>
Signed-off-by: default avatarMarc Zyngier <>
Signed-off-by: default avatarCatalin Marinas <>
parent 8d883b23
......@@ -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 (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"\
".org . - (664b-663b) + (662b-661b)\n\t" \
".org . - (662b-661b) + (664b-663b)\n"
......@@ -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"
.org . - (664b-663b) + (662b-661b)
.org . - (662b-661b) + (664b-663b)
#endif /* __ASSEMBLY__ */
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