[PATCH 3/3] arm64: alternative: Work around .inst assembler bugs

[Marc Zyngier]

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 trigger 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.
Also merge asm/alternative-asm.h into alternative.h so that we
slightly limit the duplication of this mess.

Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
 arch/arm64/include/asm/alternative-asm.h | 29 ------------------
 arch/arm64/include/asm/alternative.h     | 52 +++++++++++++++++++++++++++++---
 arch/arm64/kernel/entry.S                |  2 +-
 arch/arm64/mm/cache.S                    |  2 +-
 4 files changed, 50 insertions(+), 35 deletions(-)
 delete mode 100644 arch/arm64/include/asm/alternative-asm.h

diff --git a/arch/arm64/include/asm/alternative-asm.h b/arch/arm64/include/asm/alternative-asm.h
deleted file mode 100644
index 919a678..0000000
--- a/arch/arm64/include/asm/alternative-asm.h
+++ /dev/null
@@ -1,29 +0,0 @@
-#ifndef __ASM_ALTERNATIVE_ASM_H
-#define __ASM_ALTERNATIVE_ASM_H
-
-#ifdef __ASSEMBLY__
-
-.macro altinstruction_entry orig_offset alt_offset feature orig_len alt_len
-	.word \orig_offset - .
-	.word \alt_offset - .
-	.hword \feature
-	.byte \orig_len
-	.byte \alt_len
-.endm
-
-.macro alternative_insn insn1 insn2 cap
-661:	\insn1
-662:	.pushsection .altinstructions, "a"
-	altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f
-	.popsection
-	.pushsection .altinstr_replacement, "ax"
-663:	\insn2
-664:	.popsection
-	.if ((664b-663b) != (662b-661b))
-		.error "Alternatives instruction length mismatch"
-	.endif
-.endm
-
-#endif  /*  __ASSEMBLY__  */
-
-#endif /* __ASM_ALTERNATIVE_ASM_H */
diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index d261f01..5195cca 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h
@@ -1,6 +1,8 @@
 #ifndef __ASM_ALTERNATIVE_H
 #define __ASM_ALTERNATIVE_H
 
+#ifndef __ASSEMBLY__
+
 #include <linux/types.h>
 #include <linux/stddef.h>
 #include <linux/stringify.h>
@@ -24,7 +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"							\
@@ -37,8 +52,37 @@ 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
+
+.macro altinstruction_entry orig_offset alt_offset feature orig_len alt_len
+	.word \orig_offset - .
+	.word \alt_offset - .
+	.hword \feature
+	.byte \orig_len
+	.byte \alt_len
+.endm
+
+.macro alternative_insn insn1 insn2 cap
+661:	\insn1
+662:	.pushsection .altinstructions, "a"
+	altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f
+	.popsection
+	.pushsection .altinstr_replacement, "ax"
+663:	\insn2
+664:	.popsection
+	 // If any of these .org fail, it means that insn1 and insn2
+	 // don't have the same lenght. 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.
+	.org	. - (664b-663b) + (662b-661b)
+	.org	. - (662b-661b) + (664b-663b)
+.endm
+
+#endif  /*  __ASSEMBLY__  */
 
 #endif /* __ASM_ALTERNATIVE_H */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 959fe87..00df926 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -21,7 +21,7 @@
 #include <linux/init.h>
 #include <linux/linkage.h>
 
-#include <asm/alternative-asm.h>
+#include <asm/alternative.h>
 #include <asm/assembler.h>
 #include <asm/asm-offsets.h>
 #include <asm/cpufeature.h>
diff --git a/arch/arm64/mm/cache.S b/arch/arm64/mm/cache.S
index 2560e1e..70a79cb 100644
--- a/arch/arm64/mm/cache.S
+++ b/arch/arm64/mm/cache.S
@@ -22,7 +22,7 @@
 #include <linux/init.h>
 #include <asm/assembler.h>
 #include <asm/cpufeature.h>
-#include <asm/alternative-asm.h>
+#include <asm/alternative.h>
 
 #include "proc-macros.S"
 
-- 
2.1.4