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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. Rich Felker‏ @RichFelker Jan 26
      Replying to @RichFelker @halvarflake

      Moreover OOOE can be made safe, at low or no perf cost, if you're careful about cache side effects. Vendors traded marginal perf/$ gain for sloppiness that compromised safety.

      1 reply 0 retweets 10 likes
    2. Watson Ladd‏ @WatsonLadd Jan 26
      Replying to @RichFelker @halvarflake

      Show me. And performance needed for better energy usage, doing more work.

      1 reply 0 retweets 0 likes
    3. Rich Felker‏ @RichFelker Jan 26
      Replying to @WatsonLadd @halvarflake

      Naive approach which likely works (but needs verification): never fetch cache lines speculatively, only speculate with already-in-cache data.

      1 reply 0 retweets 3 likes
    4. Watson Ladd‏ @WatsonLadd Jan 26
      Replying to @RichFelker @halvarflake

      You said minimal performance impact. That won't be true. Hiding latency of cache miss by speculating on addresses crucial

      4 replies 0 retweets 3 likes
    5. curious_corn‏ @curious_corn Jan 27
      Replying to @WatsonLadd @RichFelker @halvarflake

      Tagging cache units with branch IDs and flush as branches become invalid?

      2 replies 0 retweets 0 likes
    6. Siddhesh Poyarekar‏ @siddhesh_p Jan 27
      Replying to @curious_corn @WatsonLadd and

      Flushing is pointless because the speculative load will invalidate the attackers cache line anyway, letting her know which line was loaded. We need a hidden cache that can somehow be made 'visible' when a speculated branch is taken. ARM spec allows hidden cache levels for e.g.

      1 reply 0 retweets 1 like
    7. Rich Felker‏ @RichFelker Jan 27
      Replying to @siddhesh_p @curious_corn and

      I think it's also possible to speculatively fetch to visible cache as long as the address to fetch is a retired result, not speculative, but not 100% sure.

      1 reply 0 retweets 2 likes
    8. Siddhesh Poyarekar‏ @siddhesh_p Jan 27
      Replying to @RichFelker @curious_corn and

      It's definitely better than completely giving up on speculation, although I believe that was a necessary first step. The approach of putting in speculation barriers seems kinda naive though given the amount of understanding the average dev has even of memory barriers.

      1 reply 1 retweet 0 likes
    9. Rich Felker‏ @RichFelker Jan 27
      Replying to @siddhesh_p @curious_corn and

      Absolutely. Any approach that requires work by the programmer or compiler is fundamentally bogus.

      2 replies 0 retweets 0 likes
    10. Watson Ladd‏ @WatsonLadd Jan 27
      Replying to @RichFelker @siddhesh_p and

      If compiler can do it, why is it bogus? We require compilers to allocate registers.

      2 replies 0 retweets 0 likes
      Rich Felker‏ @RichFelker Jan 27
      Replying to @WatsonLadd @siddhesh_p and

      Because that's a fundamental change to the ISA tgat renders existing software unsupported. A new ISA could require it, of course, but that seems dangerous & error-prone.

      12:16 PM - 27 Jan 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Watson Ladd‏ @WatsonLadd Jan 27
          Replying to @RichFelker @siddhesh_p and

          Why is that "dangerous and error-prone"? Lots of CPUs only have one trust boundary. Why do we need to make fix only hardware?

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Jan 27
          Replying to @WatsonLadd @siddhesh_p and

          Because people sometimes write asm by hand & are likely to get it wrong. But nobody's making a new ISA right now anyway. RV is already spec'd & in use.

          1 reply 0 retweets 0 likes
        4. Watson Ladd‏ @WatsonLadd Jan 27
          Replying to @RichFelker @siddhesh_p and

          You can add speculation barriers where they are needed.

          1 reply 0 retweets 0 likes
        5. Rich Felker‏ @RichFelker Jan 27
          Replying to @WatsonLadd @siddhesh_p and

          Again, adding them to an existing ISA is pointless because you're not fixing the bug - that you broke the original ISA contract.

          1 reply 0 retweets 0 likes
        6. Watson Ladd‏ @WatsonLadd Jan 27
          Replying to @RichFelker @siddhesh_p and

          We've got an actual security issue: it doesn't matter whether we change the ISA contact+modify software to fix. What matters is it gets fixed. And lots of machines won't have this issue.

          1 reply 0 retweets 0 likes
        7. Rich Felker‏ @RichFelker Jan 27
          Replying to @WatsonLadd @siddhesh_p and

          Yes it does matter. You really have no idea what you're talking about.

          1 reply 0 retweets 0 likes
        8. Rich Felker‏ @RichFelker Jan 27
          Replying to @RichFelker @WatsonLadd and

          Here "changing the ISA" means "declaring all existing binaries AND TOOLING for the old ISA deprecated and unusable".

          2 replies 0 retweets 0 likes
        9. Watson Ladd‏ @WatsonLadd Jan 27
          Replying to @RichFelker @siddhesh_p and

          Could you be specific about what breaks? For most machines in the world only the browser's JIT engine needs a change. For virtualized servers the hypervisor.

          1 reply 0 retweets 0 likes
        10. 2 more replies

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