Subject: [PATCH v3 3/3] powerpc/64s: feature: Work around inline asm issues

From: Bill Wendling <[email-masked]>
Date: Fri, 20 Nov 2020 14:40:34 -0800

The clang toolchain treats inline assembly a bit differently than
straight assembly code. In particular, inline assembly doesn't have the
complete context available to resolve expressions. This is intentional
to avoid divergence in the resulting assembly code.

We can work around this issue by borrowing a workaround done for ARM,
i.e. 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
explicitly forbidden.

Signed-off-by: Bill Wendling <[email-masked]>
---
 arch/powerpc/include/asm/feature-fixups.h | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/include/asm/feature-fixups.h b/arch/powerpc/include/asm/feature-fixups.h
index b0af97add751..f81036518edb 100644
--- a/arch/powerpc/include/asm/feature-fixups.h
+++ b/arch/powerpc/include/asm/feature-fixups.h
@@ -36,6 +36,18 @@ label##2:						\
 	.align 2;					\
 label##3:
 
+/*
+ * If the .org directive fails, it means that the feature instructions
+ * are smaller than the alternate instructions. This used to be written
+ * as
+ *
+ * .ifgt (label##4b-label##3b) - (label##2b-label##1b)
+ *      .error "Feature section else case larger than body"
+ * .endif
+ *
+ * but clang's assembler complains about the expression being non-absolute
+ * when the code appears in an inline assembly statement.
+ */
 #define MAKE_FTR_SECTION_ENTRY(msk, val, label, sect)		\
 label##4:							\
 	.popsection;						\
@@ -48,12 +60,9 @@ label##5:							\
 	FTR_ENTRY_OFFSET label##2b-label##5b;			\
 	FTR_ENTRY_OFFSET label##3b-label##5b;			\
 	FTR_ENTRY_OFFSET label##4b-label##5b;			\
-	.ifgt (label##4b- label##3b)-(label##2b- label##1b);	\
-	.error "Feature section else case larger than body";	\
-	.endif;							\
+	.org . - ((label##4b-label##3b) > (label##2b-label##1b)); \
 	.popsection;
 
-
 /* CPU feature dependent sections */
 #define BEGIN_FTR_SECTION_NESTED(label)	START_FTR_SECTION(label)
 #define BEGIN_FTR_SECTION		START_FTR_SECTION(97)
-- 
2.29.2.454.gaff20da3a2-goog