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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 19 Jan 2016

      On the design of the SYSENTER Intel syscall instruction, and how the elegant idea fell apart https://groups.google.com/forum/m/#!topic/comp.arch/CjDs4MJCBow%5B1-25%5D …pic.twitter.com/VIZlsh87cE

      5 replies 21 retweets 33 likes
    2. Rich Felker‏ @RichFelker 19 Jan 2016
      Replying to @FioraAeterna

      @FioraAeterna Why does anyone care about SYSENTER being 2 vs 15 cycles when kernel entry/exit overhead is 500-1000 cycles? :-(

      1 reply 1 retweet 3 likes
    3. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 19 Jan 2016
      Replying to @RichFelker

      @RichFelker because a lot of common syscalls, like say fetching time, require no security checks and can be done in a few cycles

      4 replies 0 retweets 0 likes
    4. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 19 Jan 2016
      Replying to @FioraAeterna

      @RichFelker presumably someone had a dream for making syscalls less horrible and the dream slowly shattered

      1 reply 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker 19 Jan 2016
      Replying to @FioraAeterna

      @FioraAeterna My view is just that software arch., not hw/insn timings, is the overwhelmingly dominating factor limiting syscall perf.

      1 reply 0 retweets 2 likes
    6. Rich Felker‏ @RichFelker 19 Jan 2016
      Replying to @RichFelker

      @FioraAeterna One big problem is that the Linux syscall ABI makes all registers call-saved, so they have to save/restore any kernel may use.

      2 replies 0 retweets 2 likes
    7. Rich Felker‏ @RichFelker 19 Jan 2016
      Replying to @RichFelker

      @FioraAeterna If everything were call-clobbered that would help. But sheer over-long code paths are also a big part of the problem...

      1 reply 0 retweets 0 likes
    8. Vertigo ` MFF‏ @SamuelAFalvoII 19 Jan 2016
      Replying to @RichFelker

      @RichFelker @FioraAeterna Caller or callee, doesn't matter; used registers have to be saved.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker 19 Jan 2016
      Replying to @SamuelAFalvoII

      @SamuelAFalvoII @FioraAeterna Caller doesn't have to save any ABI-call-clobbered registers it's not currently using. That's almost all.

      11:35 AM - 19 Jan 2016
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Vertigo ` MFF‏ @SamuelAFalvoII 19 Jan 2016
          Replying to @RichFelker

          @RichFelker @FioraAeterna But in POSIX, anytime you return from a system call, you might have to dispatch to a signal handler, no?

          2 replies 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker 19 Jan 2016
          Replying to @SamuelAFalvoII

          @SamuelAFalvoII @FioraAeterna If the syscall was already entitled to clobber those regs, signal handler is entitled to just save junk.

          2 replies 0 retweets 0 likes
        4. Vertigo ` MFF‏ @SamuelAFalvoII 19 Jan 2016
          Replying to @RichFelker

          @RichFelker @FioraAeterna Fair enough; another question arises though: context switches. Many OSes use syscall returns to switch contexts.

          2 replies 0 retweets 0 likes
        5. Rich Felker‏ @RichFelker 19 Jan 2016
          Replying to @SamuelAFalvoII

          @SamuelAFalvoII @FioraAeterna Again, if you switch contexts at syscall time & syscall clobbers regs, those regs don't need to be saved.

          1 reply 0 retweets 0 likes
        6. Rich Felker‏ @RichFelker 19 Jan 2016
          Replying to @RichFelker

          @SamuelAFalvoII @FioraAeterna Registers only need to be saved when they're meaningful to the (asynchronously) interrupted task.

          1 reply 0 retweets 0 likes
        7. Vertigo ` MFF‏ @SamuelAFalvoII 19 Jan 2016
          Replying to @RichFelker

          @RichFelker @FioraAeterna There's no way to know what registers are needed by the (asynchronously) interrupted task.

          1 reply 0 retweets 0 likes
        8. Rich Felker‏ @RichFelker 19 Jan 2016
          Replying to @SamuelAFalvoII

          @SamuelAFalvoII @FioraAeterna Right. That's why for async interruption (e.g. timer interrupt), you have to save all. Doesn't apply @ syscall

          0 replies 0 retweets 1 like
        9. End of conversation

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