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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. Joe Fitz‏ @securelyfitz Jan 5

      Thread time! Why can't they just quickly patch #meltdown or #spectre and push out another cpu? Why could it possibly take years? Why don't they use AGILE or x/y/z? Lots of reasons: (note: my goal is not to criticize chip manufacturers - it's to defend the constraints they have)

      18 replies 481 retweets 657 likes
      Show this thread
    2. Rich Felker‏ @RichFelker Jan 5
      Replying to @securelyfitz

      All of this is why you're supposed to make chicken bits to disable dubious, dangerous features like the sort of speculative execution that's at fault here.

      1 reply 2 retweets 9 likes
    3. Rich Felker‏ @RichFelker Jan 5
      Replying to @RichFelker @securelyfitz

      Failure to provide a way to turn off the unsafe features is what's irresponsible here; if they'd done is the safe & responsible way, a microcode update or even flipping a bit in an already-exposed MSR would have fixed it.

      2 replies 2 retweets 6 likes
    4. Joe Fitz‏ @securelyfitz Jan 5
      Replying to @RichFelker

      I have to believe there's a chicken bit for turning off speculative execution. But why would you want that, when the available OS level fixes probably provide an order of magnitude better performance? Helping that effort is pretty responsible IMHO.

      1 reply 1 retweet 14 likes
    5. Rich Felker‏ @RichFelker Jan 5
      Replying to @securelyfitz

      There are no OS level fixes for the most serious problem which is P0's "variant 1", and can't be. It doesn't cross OS/cpu privilege domains.

      2 replies 3 retweets 6 likes
    6. Joe Fitz‏ @securelyfitz Jan 5
      Replying to @RichFelker

      Remember the iphone battery deal last month? Imagine the outcry if Intel pushed patches that dropped your cpu to 5 % of it's current performance. I think others might value the trade-off differently from you.

      2 replies 1 retweet 15 likes
    7. Jonathan M Henson‏ @JonathanMHenson Jan 6
      Replying to @securelyfitz @RichFelker

      I’m trying to consider how most the internet performance would have plummeted if AWS or Azure had just turned of speculative execution. Almost immediately any infrastructure within 30% of their capacity planning would have browned out. An hour later the death spiral starts.

      1 reply 0 retweets 0 likes
    8. Joe Fitz‏ @securelyfitz Jan 6
      Replying to @JonathanMHenson @RichFelker

      Entirely turning off speculative execution is not a 30% hit. More realistically it would be a 90+% hit. That's why it's not a viable solution.

      2 replies 0 retweets 1 like
      Rich Felker‏ @RichFelker Jan 6
      Replying to @securelyfitz @JonathanMHenson

      Yes, it'd likely be like the P4 disaster with deep pipelines constantly getting flushed.

      10:50 AM - 6 Jan 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Rich Felker‏ @RichFelker Jan 6
          Replying to @RichFelker @securelyfitz @JonathanMHenson

          But maybe not as bad. Modern Intel chips are nowhere near as bad as P4 under fully unpredictable load (effectively random branches).

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Jan 6
          Replying to @RichFelker @securelyfitz @JonathanMHenson

          So a typical hit of "only" 30% might be realistic.

          3 replies 0 retweets 0 likes
        4. Joe Fitz‏ @securelyfitz Jan 6
          Replying to @RichFelker @JonathanMHenson

          I like the optimism, but we've got decades of compilers optimizing to hint speculative execution instead of optimizing the code itself. It'd be nowhere as bad as P4 since the pipeline is shorter, but you're talking about a full stall, flush, & waiting for _every_single_branch_

          3 replies 0 retweets 1 like
        5. Rich Felker‏ @RichFelker Jan 6
          Replying to @securelyfitz @JonathanMHenson

          Execution stall, yes, but decode/pipeline-fill can still happen speculatively. No idea if the chip has a way to disable execution w/o disabling decode/fill tho.

          1 reply 0 retweets 1 like
        6. Jonathan M Henson‏ @JonathanMHenson Jan 6
          Replying to @RichFelker @securelyfitz

          Wouldn’t an execution stall with hyper threading still hide a new TLB miss? For example, speculative execution isn’t that much help if you’re stalled anyways. So increasing the miss rate may not affect the performance that dramatically in that case.

          1 reply 0 retweets 0 likes
        7. Jonathan M Henson‏ @JonathanMHenson Jan 6
          Replying to @JonathanMHenson @RichFelker @securelyfitz

          Whereas if I wrote “perfect” code with no stalls, the instruction fetch would be dramatically more noticeable, and this is what scares me. Network hardware, system level processes, etc... are usually optimized this way and they are the most critical.

          1 reply 0 retweets 1 like
        8. Jonathan M Henson‏ @JonathanMHenson Jan 6
          Replying to @JonathanMHenson @RichFelker @securelyfitz

          These processes slowing down by even 10% is an internet level outage in the making.

          1 reply 0 retweets 1 like
        9. Jonathan M Henson‏ @JonathanMHenson Jan 6
          Replying to @JonathanMHenson @RichFelker @securelyfitz

          The latency would propagate exponentially throughout the internet going up the levels of abstraction. Higher dns latency, slower tls/crypto, drop in throughout, then the latency of high-level services would increase by an order of magnitude, then the applications depending on it

          1 reply 0 retweets 1 like
        10. 4 more replies

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