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>
" .byte 662b-661b\n" /* source len */ \
" .byte 664f-663f\n" /* replacement len */
" .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" \
#define ALTERNATIVE(oldinstr, newinstr, feature) \
"661:\n\t" \
oldinstr "\n" \
newinstr "\n" \
"664:\n\t" \
".popsection\n\t" \
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"
.pushsection .altinstr_replacement, "ax"
663: \insn2
664: .popsection
.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__ */
.endm
#endif /* __ASSEMBLY__ */