linux-kernel - Re: [PATCH] riscv: require alternatives framework when selecting FPU support

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Date:   Thu, 23 Mar 2023 16:56:24 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     Andrew Jones <ajones@...tanamicro.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        Conor Dooley <conor@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Heiko Stuebner <heiko@...ech.de>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        regressions@...mhuis.info, regressions@...ts.linux.dev
Subject: Re: [PATCH] riscv: require alternatives framework when selecting FPU
 support

On Thu, Mar 23, 2023 at 02:49:34PM +0000, Conor Dooley wrote:
> This would requiring picking up your patch Jason, but with an
> "if !XIP_KERNEL" added to the select.

So the risk of making this all work is that we wind up forgetting to add
`select alternatives if !xip` to various places that need it (fpu, kvm,
maybe others? future others?), because it appears to work, thanks to the
code in your patch.

But making it work is also probably a good thing, since we obviously
want the fpu and maybe other things to work on xip kernels.

So maybe we should get rid of the CONFIG_RISCV_ALTERNATIVES knob
entirely, making it "always enabled", and then conditonalize the
alternatives code to BUILD_BUG_ON when called with CONFIG_XIP_KERNEL=y.
Then, this build bug will get hit immediately by
riscv_has_extension_*(), which will then require your patch, which can
run in a `if (IS_ENABLED(XIP_KERNEL))` block or similar.

The result of that will be:
- !xip kernels properly use the fast riscv_has_extension_*() code and
  any alternatives code needed, since it's always selected.
- xip kernels get a BUILD_BUG_ON if they use any alternatives-based code
  that doesn't have a xip fallback yet.

What do you think of that approach?

A "lighter weight" version of that approach would be to just remove all of
the `select RISCV_ALTERNATIVES` lines, and instead make
RISCV_ALTERNATIVES specify `default !XIP_KERNEL`. That would more or
less amount to the above too, though with weirder error cases.

Jason