On Fri, 26 Feb 2016, Mathieu Desnoyers wrote: > ----- On Feb 26, 2016, at 11:29 AM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote: > > Right. There is no point in having two calls and two update mechanisms for a > > very similar purpose. > > > > So let userspace have one struct where cpu/seq and whatever is required for > > rseq is located and flag at register time which parts of the struct need to be > > updated. > > If we put both cpu/seq/other in that structure, why not plan ahead and make > it extensible then ? > > That looks very much like the "Thread-local ABI" series I posted last year. > See https://lkml.org/lkml/2015/12/22/464 > > Here is why I ended up introducing the specialized "getcpu_cache" system call > rather than the "generic" system call (quote from the getcpu_cache changelog): > > Rationale for the getcpu_cache system call rather than the thread-local > ABI system call proposed earlier: > > Rather than doing a "generic" thread-local ABI, specialize this system > call for a cpu number cache only. Anyway, the thread-local ABI approach > would have required that we introduce "feature" flags, which would have > ended up reimplementing multiplexing of features on top of a system > call. It seems better to introduce one system call per feature instead. > > If everyone end up preferring that we introduce a system call that implements > many features at once, that's indeed something we can do, but I remember > being told in the past that this is generally a bad idea. It's a bad idea if you mix stuff which does not belong together, but if you have stuff which shares a substantial amount of things then it makes a lot of sense. Especially if it adds similar stuff into hotpathes. > For one thing, it would make the interface more cumbersome to deal with > from user-space in terms of feature detection: if we want to make this > interface extensible, in addition to check -1, errno=ENOSYS, userspace > would have to deal with a field containing the length of the structure > as expected by user-space and kernel, and feature flags to see the common > set of features supported by kernel and user-space. > > Having one system call per feature seems simpler to handle in terms of > feature availability detection from a userspace point of view. That might well be, but that does not justify two fastpath updates, two seperate pointers to handle, etc .... Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
