Re: [PATCH] riscv: deprecate CONFIG_MMU=n

From: Conor Dooley <conor@kernel.org>
To: Charles Lohr <lohr85@gmail.com>
Cc: "Clément Léger" <cleger@rivosinc.com>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	"Conor Dooley" <conor.dooley@microchip.com>,
	"Andrew Jones" <ajones@ventanamicro.com>,
	"Damien Le Moal" <dlemoal@kernel.org>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Atish Patra" <atishp@rivosinc.com>
Subject: Re: [PATCH] riscv: deprecate CONFIG_MMU=n
Date: Mon, 26 Feb 2024 21:29:59 +0000	[thread overview]
Message-ID: <20240226-jolt-earwig-91c7b0eb868f@spud> (raw)
In-Reply-To: <CAGu26P_v9FjYq9Bncvfd-dBhdHQevvN3HpO1nqjA2hYFCpG7hg@mail.gmail.com>

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On Mon, Feb 26, 2024 at 11:00:41AM -0800, Charles Lohr wrote:
> WOAH! Please DO NOT deprecate NOMMU. I use the NOMMU build constantly
> and NOMMU Linux on RISC-V is the avenue used by many FPGA soft cores
> for Linux, as well as some limited systems.
> 
> I get new copies of the kernel when there are releases and test them
> frequently to make sure everything is still working as expected.

That is great - it is good to know that people are actively testing.
I was aware that a lot of the soft core folks did run nommu kernels (and
I know some do use XIP also) but everything I ever saw was running on
old kernels (5.x).

> On Mon, Feb 26, 2024 at 8:03 AM Conor Dooley <conor@kernel.org> wrote:
> >
> > On Mon, Feb 26, 2024 at 04:25:24PM +0100, Clément Léger wrote:
> > > I guess I could also mark XIP as deprecated.
> >
> > I'm not so sure, people recently added XIP support to QEMU (and sent
> > kernel fixes in December). XIP is also not nearly as much of a problem
> > to support, there's far less that it does differently, the main barrier
> > was the inability to test it which is no longer the case.
> > That said, XIP is gonna kill itself off I feel as it does not support
> > runtime patching and therefore is extremely limited on extensions, given
> > we use alternatives for all of that (although I suppose if someone has a
> > usecase they could make nasty macros worse and implement a compiletime
> > switch in the alternatives too).

> For us we just don't care about XIP. I mean if someone did push it
> through to fruition, I'd also test and use it, but I urge you please
> do not deprecate this.

XIP does work. What I was talking about here was supporting something
"fancier" than rv{32,64}imafdc.

> While it's sometimes needed a bit of a
> creative build to get everything working, I've never needed to patch
> anything in the kernel beyond patching in a custom console for serial
> output.
> 
> I am happy to discuss the possibility of me and or one of the other
> RISC-V soft (FPGA) core people stepping up to try to be more active,
> but so far we've just been very well serviced by the current NOMMU
> Linux setup.

Most of the issues aren't with nommu actually working, it is the extra
effort in development as it has to be accounted for. I would estimate
that 2/3 of the build issues I report on this list are nommu. The
best thing that you can do to ensure support for things you use is:
a) scream when someone wants to remove it
b) actively let people know you're using it

Seems like you're doing a) but maybe getting someone that provides
Tested-bys whenever you test the releases would be good.

Cheers,
Conor.

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