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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! I'm porting Linux to Apple Silicon Macs at @AsahiLinux. http://patreon.com/marcan  | http://github.com/sponsors/marcan 

Tokyo, Japan
marcan.st
Joined May 2009

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    1. Rich Felker‏ @RichFelker 11 May 2020
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      Replying to @marcan42 @MissingClara

      IOMMU, or just not having bus mastering. But yeah. Problem is none of their IOMMUs work. They don't fail closed.

      1 reply 0 retweets 2 likes
    2. Dr. Stephen Checkoway‏ @stevecheckoway 11 May 2020
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      Replying to @RichFelker @marcan42 @MissingClara

      Why is that? I haven't looked at how one actually programs an IOMMU. Is it a fundamental limitation or some sort of "we can't break existing devices relying on being able to read/write anywhere"?

      2 replies 0 retweets 0 likes
    3. Rich Felker‏ @RichFelker 11 May 2020
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      Replying to @stevecheckoway @marcan42 @MissingClara

      AIUI it's disabled by default until the OS enables it, bypassable in various ways, and not supported by lots of drivers so OS ends up turning it partially or fully off...

      2 replies 0 retweets 1 like
    4. Rich Felker‏ @RichFelker 11 May 2020
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      Replying to @RichFelker @stevecheckoway and

      Also even if a driver uses it, it can use it wrong by sharing memory containing pointers into another memory space (eg kernel memory) that device can then clobber.

      1 reply 0 retweets 1 like
    5. Rich Felker‏ @RichFelker 11 May 2020
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      Replying to @RichFelker @stevecheckoway and

      Right OS model is not to allow drivers to setup their own IOMMU mappings or DMA buffers, force everything through permanent DMA bounce buffers owned by kernel in fixed ranges.

      1 reply 0 retweets 0 likes
    6. Hector Martin‏ @marcan42 11 May 2020
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      Replying to @RichFelker @stevecheckoway @MissingClara

      That is not a reasonable approach. It has too much performance cost. It wouldn't work in any sensible way for e.g. graphics or high performance networking.

      1 reply 0 retweets 2 likes
    7. Rich Felker‏ @RichFelker 12 May 2020
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      Replying to @marcan42 @stevecheckoway @MissingClara

      Relevant to 1% of systems and the other 99% have to suffer insecure architecture for it.

      1 reply 0 retweets 0 likes
    8. Hector Martin‏ @marcan42 12 May 2020
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      Replying to @RichFelker @stevecheckoway @MissingClara

      You do realize that cloud is a huge market and they care about network performance, and that gamers are a rather important market and they care about graphics performance, right? Sorry, no, your approach is not viable in the general case.

      1 reply 0 retweets 1 like
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      Hector Martin‏ @marcan42 12 May 2020
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      Replying to @HarperMitchell @RichFelker and

      Isn't the whole point of all those high perf network architectures *precisely* to avoid dumb copies and instead directly map userspace packet buffers to the NIC?

      12:16 AM - 12 May 2020
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      • 黒田 東彦 stan
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        1. 黒田 東彦 stan‏ @HarperMitchell 12 May 2020
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          Replying to @marcan42 @RichFelker and

          Oh wait I skipped a "not" in that sentence, you are absolutely right

          0 replies 0 retweets 0 likes
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        1. Rich Felker‏ @RichFelker 12 May 2020
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          Replying to @marcan42 @HarperMitchell and

          Yes but it's irrelevant to anything but routers, middleboxes, and Netflix/YouTube. No client endpoint and almost no non-video server endpoint would have cpu time spent on network as the bottleneck even with completely naive (& secure!) driver architecture.

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        1. the vessel of morganna‏ @astraleureka 12 May 2020
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          Replying to @marcan42 @HarperMitchell and

          yeah, at that scale the costs of context switching and buffer copies start to matter. completely out of the realm of average use, manual configuration and operation should be assumed

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