x86: Adjust asm constraints in atomic64 wrappers

Eric pointed out overly restrictive constraints in atomic64_set(), but
there are issues throughout the file. In the cited case, %ebx and %ecx
are inputs only (don't get changed by either of the two low level
implementations). This was also the case elsewhere.

Further in many cases early-clobber indicators were missing.

Finally, the previous implementation rolled a custom alternative
instruction macro from scratch, rather than using alternative_call()
(which was introduced with the commit that the description of the
change in question actually refers to). Adjusting has the benefit of
not hiding referenced symbols from the compiler, which however requires
them to be declared not just in the exporting source file (which, as a
desirable side effect, in turn allows that exporting file to become a
real 5-line stub).

This patch does not eliminate the overly restrictive memory clobbers,
however: Doing so would occasionally make the compiler set up a second
register for accessing the memory object (to satisfy the added "m"
constraint), and it's not clear which of the two non-optimal
alternatives is better.

v2: Re-do the declaration and exporting of the internal symbols.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/4F19A2A5020000780006E0D9@nat28.tlf.novell.com
Cc: Luca Barbieri <luca@luca-barbieri.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 37ad100a221..49331bedc15 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -145,6 +145,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
  */
 #define ASM_OUTPUT2(a...) a
 
+/*
+ * use this macro if you need clobbers but no inputs in
+ * alternative_{input,io,call}()
+ */
+#define ASM_NO_INPUT_CLOBBER(clbr...) "i" (0) : clbr
+
 struct paravirt_patch_site;
 #ifdef CONFIG_PARAVIRT
 void apply_paravirt(struct paravirt_patch_site *start,