* Kees Cook <keescook@xxxxxxxxxxxx> wrote: > 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). Ok, that's pretty convincig - I'd suggest including a cicles row in the table instead of an instructions row: number of instructions is indeed slightly misleading in this case. Thanks, Ingo
