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

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Date:   Wed, 09 Mar 2022 23:34:25 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     michael@...haelkloos.com, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject:     Re: [PATCH] riscv: Work to remove kernel dependence on the M-extension

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).