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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. CopperheadOS‏ @CopperheadOS Apr 26
      Replying to @CopperheadOS @whitequark

      Can look up guides specific to the CPU to figure out what a sane initial target / voltage for it is. Auto voltage probably uses too much (tuned to work with even terrible CPUs and it varies a lot) but might as well start with that for simplicity, can drop it manually later.

      1 reply 0 retweets 0 likes
    2. whitequark‏ @whitequark Apr 26
      Replying to @CopperheadOS

      let's say I'm trying to find the lowest Vcore at which the system is stable, what is the criterion of "stable"?

      1 reply 0 retweets 0 likes
    3. CopperheadOS‏ @CopperheadOS Apr 26
      Replying to @whitequark

      The lowest value where the mprime stress test can run for hours without something going wrong, and then adding a bit more to be safe. Can also be a good idea to test heat / stability with fan speed crippled a bit so it can cope with hotter room temperatures, etc.

      1 reply 0 retweets 1 like
    4. CopperheadOS‏ @CopperheadOS Apr 26
      Replying to @CopperheadOS @whitequark

      The mprime torture tests put way more load on the CPU than you're ever going to do in practice. Can reach a temperature near ~85C with those tests and then end up with ~60C as the 100% load temperature for a mostly scalar workload.

      1 reply 0 retweets 2 likes
    5. CopperheadOS‏ @CopperheadOS Apr 26
      Replying to @CopperheadOS @whitequark

      There are AVX / AVX2 offset settings to configure the frequency drop for AVX and AVX2 though. Can help reduce that massive gap for scalar vs. vector work to reach higher speeds for scalar / SSE workloads.

      1 reply 0 retweets 0 likes
    6. whitequark‏ @whitequark Apr 26
      Replying to @CopperheadOS

      what about disabling AVX? if I'm just running compilers I'm just hitting vmovs in glibc, and it doesn't seem like AVX would win much here?

      1 reply 0 retweets 0 likes
    7. CopperheadOS‏ @CopperheadOS Apr 26
      Replying to @whitequark

      Not sure if it's possible to truly disable it so that applications can't use the instructions at all. You should probably just configure a large AVX offset so that it's stable, then disable it if desired so that nothing gets hit by the large AVX frequency offset.

      1 reply 0 retweets 0 likes
    8. whitequark‏ @whitequark Apr 26
      Replying to @CopperheadOS

      sure it's possible, it's even trivial. boot with clearcpuid=156

      2 replies 0 retweets 2 likes
    9. Rich Felker‏ @RichFelker Apr 26
      Replying to @whitequark @CopperheadOS

      It fundamentally needs to be possible, because an OS that doesn't support it (at context switch) has to be able to prevent userspace from using it.

      1 reply 0 retweets 0 likes
    10. CopperheadOS‏ @CopperheadOS Apr 26
      Replying to @RichFelker @whitequark

      It needs to be able to communicate that it's unavailable so that userspace doesn't try to use it. It's good if it's possible to make it trap instead of doing anything (needed for this use case to be sane) but it's communicating it that matters for making userspace not break.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Apr 26
      Replying to @CopperheadOS @whitequark

      Both are needed. If you can't make it trap, processes that shouldn't be able to communicate can use it as malicious covert channel.

      12:55 PM - 26 Apr 2018
      • 1 Like
      • CopperheadOS
      1 reply 0 retweets 1 like
        1. New conversation
        2. whitequark‏ @whitequark Apr 26
          Replying to @RichFelker @CopperheadOS

          now that I look at it, clearcpuid only seems to affect /proc/cpuinfo, cpuid(1) still shows AVX as enabled... weird.

          1 reply 0 retweets 0 likes
        3. CopperheadOS‏ @CopperheadOS Apr 26
          Replying to @whitequark @RichFelker

          Maybe noxsave works properly since it seems like it must for the reason @RichFelker pointed out.

          2 replies 0 retweets 0 likes
        4. whitequark‏ @whitequark Apr 26
          Replying to @CopperheadOS @RichFelker

          I've rebooted with noxsave and AVX is still present in cpuid outputs (and I confirmed that the kernel uses fxsave via dmesg). weird.

          1 reply 0 retweets 0 likes
        5. CopperheadOS‏ @CopperheadOS Apr 26
          Replying to @whitequark @RichFelker

          It traps if it's actually used though, right?

          1 reply 0 retweets 0 likes
        6. whitequark‏ @whitequark Apr 26
          Replying to @CopperheadOS @RichFelker

          yeah

          1 reply 0 retweets 0 likes
        7. whitequark‏ @whitequark Apr 26
          Replying to @whitequark @CopperheadOS @RichFelker

          ah yes, I figured it out. kernel never does cpuid emulation, only kvm can do that. and xsave has a bit in CR4.

          1 reply 0 retweets 0 likes
        8. Rich Felker‏ @RichFelker Apr 26
          Replying to @whitequark @CopperheadOS

          I thought there was a MSR to mask cpuid..?

          1 reply 0 retweets 0 likes
        9. whitequark‏ @whitequark Apr 26
          Replying to @RichFelker @CopperheadOS

          I don't think Linux has any code that uses it.

          0 replies 0 retweets 0 likes
        10. End of conversation

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