On Thu, Jul 20, 2017 at 10:15 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Thu, Jul 20, 2017 at 2:11 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote: >> Could you please also create a tabulated quick-comparison of the three variants, >> of all key properties, about behavior, feature and tradeoff differences? >> >> Something like: >> >> !ARCH_HAS_REFCOUNT ARCH_HAS_REFCOUNT=y REFCOUNT_FULL=y >> >> avg fast path instructions: 5 3 10 >> behavior on overflow: unsafe, silent safe, verbose safe, verbose >> behavior on underflow: unsafe, silent unsafe, verbose unsafe, verbose >> ... >> >> etc. - note that this table is just a quick mockup with wild guesses. (Please add >> more comparisons of other aspects as well.) >> >> Such a comparison would make it easier for arch, subsystem and distribution >> maintainers to decide on which variant to use/enable. > > Sure, I can write this up. I'm not sure "safe"/"unsafe" is quite that > clean. The differences between -full and -fast are pretty subtle, but > I think I can describe it using the updated LKDTM tests I've written > to compare the two. There are conditions that -fast doesn't catch, but > those cases aren't actually useful for the overflow defense. > > As for "avg fast path instructions", do you mean the resulting > assembly for each refcount API function? I think it's going to look > something like "1 2 45", but I'll write it up. So, doing a worst-case timing of a loop of inc() to INT_MAX and then dec_and_test() back to zero, I see this out of perf: atomic 25255.114805 task-clock (msec) 82249267387 cycles 11208720041 instructions refcount-fast 25259.577583 task-clock (msec) 82211446892 cycles 15486246572 instructions refcount-full 44625.923432 task-clock (msec) 144814735193 cycles 105937495952 instructions I'll still summarize all this in the v7 series, but I think that really clarifies the differences: 1.5x more instructions in -fast, but nearly identical cycles and clock. Using -full sees a large change (as expected). -Kees -- Kees Cook Pixel Security
