On Sat, Apr 04, 2015 at 11:25:36AM +0200, Borislav Petkov wrote: > On Sat, Apr 04, 2015 at 10:36:11AM +0200, Quentin Casasnovas wrote: > > Since all of these are compile time constants, could we not use the safe > > variant on that same page? Not that I'm too worried about the signed right > > shift but heh that would be portable and should not impact performance > > anyway, so no added value in using the optimized version is there? > > Seems to work with the experimental diff below. I need to do > > -(-(x < y)) > > with the last term though as we're working with s32s. > > --- > diff --git a/arch/x86/include/asm/alternative-asm.h > b/arch/x86/include/asm/alternative-asm.h > index 44a1fc5439d3..2cb6da2716bf 100644 > --- a/arch/x86/include/asm/alternative-asm.h > +++ b/arch/x86/include/asm/alternative-asm.h > @@ -53,14 +53,14 @@ > * Shamelessly stolen and adapted from: > * http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax > */ > -#define alt_max_short(a,b) (((a) - (((a) - (b)) & (((a) - (b)) >> 15))) & > 0xffff) > +#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b))))) >
So I'm not claiming I've spent the time to fully understand this macro but it looks like it's doing the right thing on my dummy tests: http://pastebin.com/DDhtZQgX Did you also change it in the alternative.h file BTW? > .macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2 > 140: > \oldinstr > 141: > - .skip -((alt_max_short(new_len1, new_len2) - old_len) > 0) * \ > - (alt_max_short(new_len1, new_len2) - old_len),0x90 > + .skip -((alt_max_short(new_len1, new_len2) - (old_len)) > 0) * \ > + (alt_max_short(new_len1, new_len2) - (old_len)),0x90 > 142: Good catch for the missing parenthesis! > > > > Looks good to me and I find it much easier to understand here :) > > > > Cool. Please give it more critical staring as we're under time pressure > here. > So I _think_ it's OK but it would be re-assuring if somebody else could have a look as well just in case.. :) Do you have a cleaned up version of the patch you're planning to apply on top of tip/master instead of just snippets? This way we can hammer it with different calls to ALTERNATIVE_2 and alternative_2 to check it's good? I'll have to leave soonish though.. Quentin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
