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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. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
      Replying to @DrPizza @RichFelker @arstechnica

      Spectre v1 doesn't cross protection domains, and we want correct speculations to be able to cause cache misses.

      1 reply 0 retweets 0 likes
    2. Rich Felker‏ @RichFelker Feb 21
      Replying to @DrPizza @arstechnica

      Rich Felker Retweeted Rich Felker

      Yes it does.https://twitter.com/RichFelker/status/948750800553168896 …

      Rich Felker added,

      Rich Felker @RichFelker
      Spectre is essentially the failure by cpu designers to realize that an untaken branch is fundamentally a privilege boundary.
      1 reply 0 retweets 0 likes
    3. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
      Replying to @RichFelker @arstechnica

      No it doesn't. The only architectural protection domains are the rings, to an extent different values of CR3, and SGX.

      3 replies 0 retweets 0 likes
    4. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
      Replying to @DrPizza @RichFelker @arstechnica

      Software-enforced domains (sandboxes, etc.) need software-enforced fixes.

      1 reply 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker Feb 21
      Replying to @DrPizza @arstechnica

      The scope of Spectre v1 is way bigger than things you actively think of as "sandboxes". It's unbounded attack surface. There will be clever new exploits of this for *decades* if the underlying bug is not fixed.

      2 replies 0 retweets 0 likes
    6. Rich Felker‏ @RichFelker Feb 21
      Replying to @RichFelker @DrPizza @arstechnica

      So no, I don't want correct speculations to cause cache misses. Preventing that would cost <1% perf to workloads with low to moderate cache miss rate. But I don't believe it's possible with just uc updates, needs bigger changes.

      2 replies 0 retweets 0 likes
    7. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
      Replying to @RichFelker @arstechnica

      What's your source for "<1%", and what fraction of loads qualify as "low to moderate cache miss rate"?

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker Feb 21
      Replying to @DrPizza @arstechnica

      Intuition. But you could use performance counters to measure the rate of misses and estimate the impact, I think. Just add K*M where M is # of misses and K is pipeline stall + fetch cost.

      1 reply 0 retweets 0 likes
    9. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
      Replying to @RichFelker @arstechnica

      I do not believe that intuition is a useful guide when estimating the interactions between, and impact of, changes to speculation and cache behaviour.

      1 reply 0 retweets 0 likes
    10. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
      Replying to @DrPizza @RichFelker @arstechnica

      consider also that some speculative loads are benign, e.g.: if(cond) { arr[i] += 1; } else { arr[i] -= 1; } whichever path is speculated, the load is good.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Feb 21
      Replying to @DrPizza @arstechnica

      In this case a good compiler can/should hoist the load out of the branches. So it doesn't matter if cpu can speculate it.

      12:30 PM - 21 Feb 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
          Replying to @RichFelker @arstechnica

          Not necessarily, because inc dword ptr [eax], for example, generates shorter code.

          1 reply 0 retweets 0 likes
        3. Pumpkin Fright 👨‍⚕️ 🍕‏Verified account @DrPizza Feb 21
          Replying to @DrPizza @RichFelker @arstechnica

          compare for example mov eax, dword ptr [ecx] test ebx, ebx je 1f inc eax jmp 2f 1: dec eax 2: mov dword ptr [ecx], eax test ebx, ebx je 3f inc dword ptr [ecx] jmp 4f 3: dec dword ptr [ecx] 4:

          0 replies 0 retweets 0 likes
        4. End of conversation

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