5 ms·

I'm asking out of naiveté here -- how were they (kernel maintainers) able to get the Linux kernel to support M1 with undocumented instructions?

frabert 5y ago
A kernel doesn't need to use all the instructions a CPU offers -- only the ones it needs.
- ithkuil 5y ago
  If the extra instructions also operate in extra state (e.g. extra registers) a kernel needs to know about their existence so it can correctly save and restore state on context switches
  - my123 5y ago
    Not necessarily/really, the custom extensions still need to be enabled by the kernel before they can be used. As such, it isn’t actually an issue.
  - ndesaulniers 5y ago
    You're confusing MSRs (which don't have to be saved/restored on context switch) with GPRs (which do).
    - saagarjha 5y ago
      Apple reserves certain MSRs that apply on a per-process basis, and thus must be applied on context switch.
    - ithkuil 5y ago
      Well if there is a truly undocumented extension, how do I know it doesn't come with it's own registers (e.g. like a floating point unit does)
- Someone 5y ago
  It doesn’t need to use them, but it must be aware of them, insofar as they may introduce security problems. As an example, if the kernel doesn’t know of DMA channels, and it requires setup code to prevent user-level code from using them to copy across process boundaries, the kernel will run fine, but have glaring security problems.
  - surajrmal 5y ago
    What dma channels doesn't require mapping registers into the user space process to work? There aren't usually magic instructions you have to opt into disabling as far as I know.
  - im3w1l 5y ago
    Those are not security problems, they are insecurity features readjusts tinfoil.
heavyset_go 5y ago
I'm assuming they aren't using them or they've reverse engineered the ones they need to use.
kevingadd 5y ago
The undocumented instructions aren't required in order to use the hardware
R0b0t1 5y ago
Don't use them. The instructions necessary to run Linux are likely inherited from the normal ARM set.
deleted 5y ago
[deleted]
ducktective 5y ago
My guess: if the situation is similar to Windows laptops, they just use a subset of OEM features and provide a sub-par experience (like lack of battery usage optimizations, flaky suspend/hibernate, second-tier graphics support, etc) Now, I'm typing this on a GNU/Linux machine, but let's face it, all of the nuisances I mentioned are legit and constant source of problems in tech support forums.
mhh__ 5y ago
The instructions are (all?) for acceleration I think.
- saagarjha 5y ago
  Not all of them, but the ones that don't (GXF, memory compression, etc.) are opt-in.

frabert 5y ago

A kernel doesn't need to use all the instructions a CPU offers -- only the ones it needs.

ithkuil 5y ago
If the extra instructions also operate in extra state (e.g. extra registers) a kernel needs to know about their existence so it can correctly save and restore state on context switches
- my123 5y ago
  Not necessarily/really, the custom extensions still need to be enabled by the kernel before they can be used. As such, it isn’t actually an issue.
- ndesaulniers 5y ago
  You're confusing MSRs (which don't have to be saved/restored on context switch) with GPRs (which do).
  - saagarjha 5y ago
    Apple reserves certain MSRs that apply on a per-process basis, and thus must be applied on context switch.
  - ithkuil 5y ago
    Well if there is a truly undocumented extension, how do I know it doesn't come with it's own registers (e.g. like a floating point unit does)
Someone 5y ago
It doesn’t need to use them, but it must be aware of them, insofar as they may introduce security problems. As an example, if the kernel doesn’t know of DMA channels, and it requires setup code to prevent user-level code from using them to copy across process boundaries, the kernel will run fine, but have glaring security problems.
- surajrmal 5y ago
  What dma channels doesn't require mapping registers into the user space process to work? There aren't usually magic instructions you have to opt into disabling as far as I know.
- im3w1l 5y ago
  Those are not security problems, they are insecurity features readjusts tinfoil.

heavyset_go 5y ago

I'm assuming they aren't using them or they've reverse engineered the ones they need to use.

kevingadd 5y ago

The undocumented instructions aren't required in order to use the hardware

R0b0t1 5y ago

Don't use them. The instructions necessary to run Linux are likely inherited from the normal ARM set.

deleted 5y ago

[deleted]

ducktective 5y ago

My guess: if the situation is similar to Windows laptops, they just use a subset of OEM features and provide a sub-par experience (like lack of battery usage optimizations, flaky suspend/hibernate, second-tier graphics support, etc) Now, I'm typing this on a GNU/Linux machine, but let's face it, all of the nuisances I mentioned are legit and constant source of problems in tech support forums.

mhh__ 5y ago

The instructions are (all?) for acceleration I think.

saagarjha 5y ago
Not all of them, but the ones that don't (GXF, memory compression, etc.) are opt-in.