Chester (lamchester.delete@this.gmail.com) on October 19, 2022 12:11 pm wrote:
> Ana Rodriguez (ana_rodriguez_riano.delete@this.protonmail.com) on October 19, 2022 11:12 am wrote:
> > If someone were to build a Linux system with proprietary,
> > private (undocumented) ISA extensions, how would they
> > do it given Linux is open source? Are there any examples of this being done? Would it be possible at all?
> >
> > I got inspiration from this (https://github.com/corsix/amx) and I wondered if someone has done
> > it before on a Linux-based system. I understand a userspace library could be created to access
> > those instructions from userspace, but how would then they be implemented in the kernel? Through
> > a proprietary kernel module built using a custom compiler? Or is that not needed at all and the
> > library could just run on the processor taking advantage of the proprietary extensions?
>
> If the instructions don't create any extra state that the kernel needs to save or be aware
> of, then you could use the instructions in userspace without any modification to the kernel.
> However, if you need the kernel to save extra state, you do need a kernel change.
>
> Example of something that doesn't need a kernel change: MMX. Intel specifically aliased the MMX registers
> to the x87 ones, so an OS doesn't even need to know that MMX is supported. It saves the x87 registers on
> a context switch, and by doing so it would also save the MMX registers (because ST0-7 == MM0-7).
>
> Example of something that does need a kernel change: SSE. SSE adds a set of separate vector/FP registers
> (xmm regs). The kernel has to save and restore those on a context switch, and thus needs a change.
>
> In Apple AMX's case, they introduce an extra 80 registers, each of which are 64 bytes wide.
> The kernel needs to save and restore these on a context switch. So, you need a kernel change.

I see. Thank you for your response!

I unfortunately this is one limitation of the GPL...