5 ms·

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.