I have to admit I'm a little confused ...
1) It is entirely reasonable that Marcan and the Asahi devs would not include support for undocumented instructions like AMX in the Linux Kernel. They will undoubtedly support other undocumented accelerators (like the media engine) but as with the GPU that will be through drivers, not direct instruction support.
2) I would've thought that the very well documented and very ARM NEON instructions would be the most natural way to emulate AVX, even AVX-512.
The key part for me being the “Apple chips will almost certainly drop support for AMX in favour of SVE..”.
This would be surprising to me. I can’t imagine they didn’t know that SVE would come eventually. Would they really leave AMX for SVE? Thoughts.
3) SVE is the replacement for NEON not AMX. In fact in ARM v9 it is SVE2. The corollary in ARMv9 to AMX is
SME.
By "I can’t imagine they didn’t know that SVE would come eventually." I assume the "they" is Apple? If so, then yes, and probably for years before it was announced. I believe AMX has been reverse engineered but I do not know how close SME and AMX are. If they are similar, that wouldn't surprise me given the close relationship between ARM and Apple. But even if they aren't, Hector is right that AMX instructions could relatively easily be replaced by SME if Apple wanted to since the AMX instructions are undocumented and accessible only through Apple's Accelerate framework. Thus, the only thing that would need to be recoded is Accelerate. That is in fact the whole reason why if an ISA vendor creates its own customs extensions that ARM demands they remain undocumented.
That said we don't know when Apple will adopt ARM v9. They may choose not to do so (at least not soon) as the main upgrades relevant for them would be the Matrix and Vector extensions and they may decide that NEON and AMX serve them well enough. The other major upgrade in ARM v9 is confidential computing and while Apple is quite concerned with security on its chips, my lay person's impression is that confidential computing is mostly beneficial to server chips. This would explain why Apple has not been in a rush to adopt ARM v9 like they were with ARM v8. Still, maybe it'll come with the next generation of chips.
I dunno, I think Hector missed the mark on this prediction. SVE and AMX don't do quite the same things.
I agree that Hector has seemingly confused the two possibly due to the context of the rest of the conservation. However, AMX may disappear if Apple adopts ARM v9 but it'll be replaced by SME, not SVE2.