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As mentioned in the article, TSO is not exclusive to Apple's ARM implementation. > Some NVIDIA and Fujitsu CPUs run with TSO at all times; Apple's CPUs provid

As mentioned in the article, TSO is not exclusive to Apple's ARM implementation. > Some NVIDIA and Fujitsu CPUs run with TSO at all times; Apple's CPUs provide it as an optional feature that can be enabled at run time.

dmitrygr 10mo ago
> As mentioned in the article, TSO is not exclusive to Apple's ARM implementation. I thought I had been quite clear. I guess I'll try again even more clearly. "TSO" is three letters. It is not a spec. "We all do TSO" is as meaningful as "we all want world peace". Everyone has their own meaning for those words, and the meanings may differ significantly. Each is a memory model, and each can be called "TSO". But just like not every "John Smith" is the same person, nor is everything called "TSO" the same. Does NVIDIA's TSO order ALL reads with respect to ALL writes? Does Apple's? What does x86 do in that case? What does a Fujitsu CPU do? "TSO" does not mean the same thing to everyone just like "world peace" does not. If, for example, NVIDIA came out and said "our TSO mode complies 100% with x86 memory model and will always continue to", then Fujitsu did the same, and then (LOL) Apple also publicly promised that, then and only then would your comment make sense. As it stands, four entities use the same acronym to each mean their own thing, and you are assuming absolute equality because the three letters match. Fun story: I know FOR A FACT the answer to my above question about ordering of all reads vs all writes is not the same for x86, Apple's TSO, NVIDIA's TSO, and Fujitsu's TSO. Do you? Do you know how? Do you know how the answers might change with time and hardware revisions, given that at least Apple made no promises as to how their undocumented TSO mode works today or will work tomorrow? Exactly... One cannot build a stable f{ea,u}ture on undocumented un[der]specified hardware features.
- GeekyBear 10mo ago
  Were you aware that all the BIOS implementations used in PC compatible computers (Compaq, AMI, Phoenix, etc.) were not identical and were compatible to a greater or lesser extent with the original IBM BIOS, yet Linux somehow supported PC compatible computers? > Someone saying "it is TSO" is not documentation. Trying to re-implement what IBM's BIOS did was not documentation either. The original sets the standard, whether a given implementation is perfectly equivalent or not.
  - dmitrygr 10mo ago
    I see no further point for this discussion. Either you truly do not understand or are pretending to not understand the difference between memory models (affect literally every memory access as long as the system is powered up) and BIOS (not used once the OS is up, and thus one-time at-boot quirks handling code can work around most issues). Either way, g'day. Oh, and to answer your question, yes, quite aware, actually. I've done quite a bit of low level work over the decades, including, curiously, working in the Apple platform kernel team at the time when this TSO bit appeared.
- Dylan16807 10mo ago
  > I know FOR A FACT the answer to my above question about ordering of all reads vs all writes is not the same for x86, Apple's TSO, NVIDIA's TSO, and Fujitsu's TSO. Well of course they differ. TSO says that some reorderings are banned and some are optional, and there's a million factors that go into deciding when those options are taken. > "TSO" is three letters. It is not a spec. It's a few rules that you can depend on. Are those rules not enough to build a program on top of? The simpler you make your rules, the less spec you need. On the other end of the spectrum, a dozen specialized memory barriers need a ton of explanation.
  - dmitrygr 10mo ago
    >> "TSO" is three letters. It is not a spec. >It's a few rules that you can depend on. Until properly specified they are not "rules" but "hopes". Apple made no promises and provided no specs for their TSO mode. What makes you sure that that TSO bit on AppleM4pro acts the same as on AppleM1? That same "TSO" bit might mean yet a third thing on AppleM7megaMaxProEliteG2 in 2031. How do you know that an OS update that also updated iBoot on your Mac did not change some internal chip config MSR and now even on your AppleM4pro CPU whose TSO you understood, it acts differently due to this config bit change?
    - Dylan16807 10mo ago
      I wasn't talking about Apple's promises, I was talking about the meaning of "TSO". If you know you have TSO, you have some rules you can depend on. What's an example of something you need beyond those rules, to write correctly concurrent code?
      - dmitrygr 10mo ago
        > If you know you have TSO "If you know you have world peace" Sure, now define "total". Which accesses does that affect and which ones does it not? Is device memory included? PCIe memory? Are there ordering guarantees between mappings with different permissions? Then, define "store ordering". Does it affect loads in any way? Or simply just stores?

dmitrygr 10mo ago

> As mentioned in the article, TSO is not exclusive to Apple's ARM implementation. I thought I had been quite clear. I guess I'll try again even more clearly. "TSO" is three letters. It is not a spec. "We all do TSO" is as meaningful as "we all want world peace". Everyone has their own meaning for those words, and the meanings may differ significantly. Each is a memory model, and each can be called "TSO". But just like not every "John Smith" is the same person, nor is everything called "TSO" the same. Does NVIDIA's TSO order ALL reads with respect to ALL writes? Does Apple's? What does x86 do in that case? What does a Fujitsu CPU do? "TSO" does not mean the same thing to everyone just like "world peace" does not. If, for example, NVIDIA came out and said "our TSO mode complies 100% with x86 memory model and will always continue to", then Fujitsu did the same, and then (LOL) Apple also publicly promised that, then and only then would your comment make sense. As it stands, four entities use the same acronym to each mean their own thing, and you are assuming absolute equality because the three letters match. Fun story: I know FOR A FACT the answer to my above question about ordering of all reads vs all writes is not the same for x86, Apple's TSO, NVIDIA's TSO, and Fujitsu's TSO. Do you? Do you know how? Do you know how the answers might change with time and hardware revisions, given that at least Apple made no promises as to how their undocumented TSO mode works today or will work tomorrow? Exactly... One cannot build a stable f{ea,u}ture on undocumented un[der]specified hardware features.

GeekyBear 10mo ago
Were you aware that all the BIOS implementations used in PC compatible computers (Compaq, AMI, Phoenix, etc.) were not identical and were compatible to a greater or lesser extent with the original IBM BIOS, yet Linux somehow supported PC compatible computers? > Someone saying "it is TSO" is not documentation. Trying to re-implement what IBM's BIOS did was not documentation either. The original sets the standard, whether a given implementation is perfectly equivalent or not.
- dmitrygr 10mo ago
  I see no further point for this discussion. Either you truly do not understand or are pretending to not understand the difference between memory models (affect literally every memory access as long as the system is powered up) and BIOS (not used once the OS is up, and thus one-time at-boot quirks handling code can work around most issues). Either way, g'day. Oh, and to answer your question, yes, quite aware, actually. I've done quite a bit of low level work over the decades, including, curiously, working in the Apple platform kernel team at the time when this TSO bit appeared.
Dylan16807 10mo ago
> I know FOR A FACT the answer to my above question about ordering of all reads vs all writes is not the same for x86, Apple's TSO, NVIDIA's TSO, and Fujitsu's TSO. Well of course they differ. TSO says that some reorderings are banned and some are optional, and there's a million factors that go into deciding when those options are taken. > "TSO" is three letters. It is not a spec. It's a few rules that you can depend on. Are those rules not enough to build a program on top of? The simpler you make your rules, the less spec you need. On the other end of the spectrum, a dozen specialized memory barriers need a ton of explanation.
- dmitrygr 10mo ago
  >> "TSO" is three letters. It is not a spec. >It's a few rules that you can depend on. Until properly specified they are not "rules" but "hopes". Apple made no promises and provided no specs for their TSO mode. What makes you sure that that TSO bit on AppleM4pro acts the same as on AppleM1? That same "TSO" bit might mean yet a third thing on AppleM7megaMaxProEliteG2 in 2031. How do you know that an OS update that also updated iBoot on your Mac did not change some internal chip config MSR and now even on your AppleM4pro CPU whose TSO you understood, it acts differently due to this config bit change?
  - Dylan16807 10mo ago
    I wasn't talking about Apple's promises, I was talking about the meaning of "TSO". If you know you have TSO, you have some rules you can depend on. What's an example of something you need beyond those rules, to write correctly concurrent code?
    - dmitrygr 10mo ago
      > If you know you have TSO "If you know you have world peace" Sure, now define "total". Which accesses does that affect and which ones does it not? Is device memory included? PCIe memory? Are there ordering guarantees between mappings with different permissions? Then, define "store ordering". Does it affect loads in any way? Or simply just stores?