[PATCH 3.16 17/86] jump_label/x86: Work around asm build bug on older/backported GCCs

Ben Hutchings
2019-05-16T16:04:49Z

From: Peter Zijlstra <peterz@infradead.org>

commit d420acd816c07c7be31bd19d09cbcb16e5572fa6 upstream.

Boris reported that gcc version 4.4.4 20100503 (Red Hat
4.4.4-2) fails to build linux-next kernels that have
this fresh commit via the locking tree:

11276d5306b8 ("locking/static_keys: Add a new static_key interface")

The problem appears to be that even though @key and @branch are
compile time constants, it doesn't see the following expression
as an immediate value:

&((char *)key)[branch]

More recent GCCs don't appear to have this problem.

In particular, Red Hat backported the 'asm goto' feature into 4.4,
'normal' 4.4 compilers will not have this feature and thus not
run into this asm.

The workaround is to supply both values to the asm as immediates
and do the addition in asm.

Suggested-by: H. Peter Anvin <hpa@zytor.com>
Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/include/asm/jump_label.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -22,9 +22,9 @@ static __always_inline bool arch_static_
".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
".pushsection __jump_table, \"aw\" \n\t"
_ASM_ALIGN "\n\t"
- _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+ _ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
".popsection \n\t"
- : : "i" (&((char *)key)[branch]) : : l_yes);
+ : : "i" (key), "i" (branch) : : l_yes);

return false;
l_yes:
@@ -38,9 +38,9 @@ static __always_inline bool arch_static_
"2:\n\t"
".pushsection __jump_table, \"aw\" \n\t"
_ASM_ALIGN "\n\t"
- _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+ _ASM_PTR "1b, %l[l_yes], %c0 + %c1 \n\t"
".popsection \n\t"
- : : "i" (&((char *)key)[branch]) : : l_yes);
+ : : "i" (key), "i" (branch) : : l_yes);

return false;