Re: [PATCH] riscv: Work to remove kernel dependence on the M-extension

Palmer Dabbelt
2022-03-10T07:34:41Z

On Wed, 09 Mar 2022 23:06:22 PST (-0800), Christoph Hellwig wrote:
> Why?

I have no idea, but this has come up a few times before.

IIRC the original port had a no-M flavor (don't remember if it even made
it to the original patch submission, but it was around for a bit). We
decided to drop this under the theory that nobody would use it:
essentially, if you can afford the handful of MiB of memory required to
run Linux then you've probably got a multiplier.

If someone has hardware that lacks M and users care about running Linux
on that then I'm happy to support it. I'll still point out the
silliness of that decision, but if it's too late to change things then
I'd rather support the hardware. If it's one of these "fill out every
possible allowed ISA flavor, even if nobody has one that runs Linux"
then I don't see a reason to bother -- there's an infinite amount of
allowable RISC-V implementations, we'll all go crazy chasing them
around.

FWIW: to a first order, that applies to the no-A stuff as well (though
that got dropped because the Arm folks pointed out a way to support that
was way better than ours).