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I can read the English, I can Google the lingo, but I'm just a software guy still, and this only parses to me as "there's a bunch of memory management hardware

I can read the English, I can Google the lingo, but I'm just a software guy still, and this only parses to me as "there's a bunch of memory management hardware on a CPU that is really important to efficient virtualization, some of it standardized by ARM, some which might make it trivial to support virtualization, but no one knows which standards Apple has settled upon in their silicon". Is there a more ELI5'ish accessible walk through the context and why's behind this part of the discussion? It sounds really fascinating to me, but I'm not yet equipped to understand it well.

marcan_42 6y ago
The simple answer is that Apple builds their own CPU cores, so they have to build their own virtualization. Virtualization is not something you tack on around a CPU core, it's something that's part of a CPU core. Since Apple aren't using ARM CPUs, they can't use ARM virtualization. Anyone suggesting otherwise is confused :-) (Apple might be able to leverage off the shelf ARM technologies that might help with virtualization, but not the core feature of virtualization itself)
- saagarjha 6y ago
  They’re using ARM CPUs but not arm’s CPUs ;)
  - jki275 6y ago
    They are using the ARM ISA on their own custom silicon.
- yourapostasy 6y ago
  Thanks so much, that watered it down for me just right. So are there any tells/indications that Apple's silicon took this into consideration from the beginning, or is virtualization something they had to retrofit into the core? It would be pretty amazing if say, way back in A1 days, we could point to something in the core that indicated they already started laying the groundwork to make virtualization feasible later.
  - marcan_42 6y ago
    A1 is not a thing, and the first 64-bit Apple Silicon was the A7, so I don't expect anything to have appeared before then. Virtualization isn't terribly hard to add to a core, so they could've started thinking about it at any time. It's possible that some of their cores already support it and we just don't know; that would be a very Apple thing to do. The way virtualization on ARM works (at least the way ARM themselves implemented it; Apple could've done something differently) is that there are three execution levels: EL2 (VM), EL1 (guest kernel), and EL0 (guest userspace). So a device that supports EL2 but drops immediately to EL1 on boot to run a normal kernel (without virtualization active) would not necessarily have obvious "tells" that it supports virtualization, unless you broke into the boot process early enough to catch it in EL2. It would be interesting to break into an A11 device using the checkm8 exploit and see if there is any evidence of EL2/virtualization support on that core. Here's a fun one though: Apple CPUs did at least at one point support EL3 (that's one level higher, TrustZone), which they used for KPP: https://xerub.github.io/ios/kpp/2017/04/13/tick-tock.html https://xerub.github.io/ios/kpp/2017/04/13/tick-tock.html Which suggests they might support EL2 and virtualization too. Honestly, I can't find any trustworthy reference claiming that existing Apple Silicon supports virtualization, nor that it doesn't. For all we know it does..
    - saagarjha 6y ago
      No shipping Apple CPUs support it EL2. KPP/WatchTower was inherently racy/bypassable and has been dead for years, replaced with KTRR which is baked into the silicon itself.

marcan_42 6y ago

The simple answer is that Apple builds their own CPU cores, so they have to build their own virtualization. Virtualization is not something you tack on around a CPU core, it's something that's part of a CPU core. Since Apple aren't using ARM CPUs, they can't use ARM virtualization. Anyone suggesting otherwise is confused :-) (Apple might be able to leverage off the shelf ARM technologies that might help with virtualization, but not the core feature of virtualization itself)

saagarjha 6y ago
They’re using ARM CPUs but not arm’s CPUs ;)
- jki275 6y ago
  They are using the ARM ISA on their own custom silicon.
yourapostasy 6y ago
Thanks so much, that watered it down for me just right. So are there any tells/indications that Apple's silicon took this into consideration from the beginning, or is virtualization something they had to retrofit into the core? It would be pretty amazing if say, way back in A1 days, we could point to something in the core that indicated they already started laying the groundwork to make virtualization feasible later.
- marcan_42 6y ago
  A1 is not a thing, and the first 64-bit Apple Silicon was the A7, so I don't expect anything to have appeared before then. Virtualization isn't terribly hard to add to a core, so they could've started thinking about it at any time. It's possible that some of their cores already support it and we just don't know; that would be a very Apple thing to do. The way virtualization on ARM works (at least the way ARM themselves implemented it; Apple could've done something differently) is that there are three execution levels: EL2 (VM), EL1 (guest kernel), and EL0 (guest userspace). So a device that supports EL2 but drops immediately to EL1 on boot to run a normal kernel (without virtualization active) would not necessarily have obvious "tells" that it supports virtualization, unless you broke into the boot process early enough to catch it in EL2. It would be interesting to break into an A11 device using the checkm8 exploit and see if there is any evidence of EL2/virtualization support on that core. Here's a fun one though: Apple CPUs did at least at one point support EL3 (that's one level higher, TrustZone), which they used for KPP: https://xerub.github.io/ios/kpp/2017/04/13/tick-tock.html https://xerub.github.io/ios/kpp/2017/04/13/tick-tock.html Which suggests they might support EL2 and virtualization too. Honestly, I can't find any trustworthy reference claiming that existing Apple Silicon supports virtualization, nor that it doesn't. For all we know it does..
  - saagarjha 6y ago
    No shipping Apple CPUs support it EL2. KPP/WatchTower was inherently racy/bypassable and has been dead for years, replaced with KTRR which is baked into the silicon itself.