> On Sep 16, 2018, at 10:26 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > > As far as I can tell (i.e., as a user not a network dev), WireGuard is > an excellent piece of code, and I would like to see it merged. I also > think there is little disagreement about the quality of the proposed > algorithm implementations and the usefulness of having a set of easy > to use solid crypto primitives in addition to or complementing the > current crypto API. > > I do have some concerns over how the code is organized though: > > * simd_relax() is currently not called by the crypto routines > themselves. This means that the worst case scheduling latency is > unbounded, which is unacceptable for the -rt kernel. The worst case > scheduling latency should never be proportional to the input size. > (Apologies for not spotting that earlier) > > * Using a cute name for the crypto library [that will end up being the > preferred choice for casual use only] may confuse people, given that > we have lots of code in crypto/ already. I'd much prefer using, e.g., > crypto/lib and crypto/api (regardless of where the arch specific > pieces live) > > * I'd prefer the arch specific pieces to live under arch, but I can > live with keeping them in a single place, as long as the arch > maintainers have some kind of jurisdiction over them. I also think > there should be some overlap between the maintainership > responsibilities of the two generic pieces (api and lib). > > * (Nit) The GCC command line -include'd .h files contain variable and > function definitions so they are actually .c files. Hmm. I would suggest just getting rid of the -include magic entirely. The resulting ifdef will be more comprehensible. > * The current organization of the code puts all available (for the > arch) versions of all routines into a single module, which can only be > built in once we update random.c to use Zinc's chacha20 routines. This > bloats the core kernel (which is a huge deal for embedded systems that > have very strict boot requirements). It also makes it impossible to > simply blacklist a module if you, for instance, prefer to use the > [potentially more power efficient] scalar code over the SIMD code when > using a distro kernel. I think the module organization needs to change. It needs to be possible to have chacha20 built in but AES or whatever as a module. > > [To summarize the 4 points above, I'd much rather see a more > conventional organization where different parts are provided by > different modules. I don't think the argument that inlining is needed > for performance is actually valid, given that we have branch > predictors and static keys, and the arch SIMD code is built as > separate object files anyway] I might have agreed before Spectre :(. Unfortunately, unless we do some magic, I think the code would look something like: if (static_branch_likely(have_simd)) arch_chacha20(); ...where arch_chacha20 is a *pointer*. And that will generate a retpoline and run very, very slowly. (I just rewrote some of the x86 entry code to eliminate one retpoline. I got a 5% speedup on some tests according to the kbuild bot.) So, if we really wanted modules, we’d need a new dynamic patching mechanism. I would suggest instead adding two boot (or debugfs) options: simd=off: disables simd_get using a static branch. crypto_chacha20_nosimd: Does what it sounds like.
