linux-kernel - Re: [PATCH] arm64: rust: clean Rust 1.85.0 warning using softfloat target

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Message-ID: <20250211103333.GB8653@willie-the-truck>
Date: Tue, 11 Feb 2025 10:33:33 +0000
From: Will Deacon <will@...nel.org>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Alex Gaynor <alex.gaynor@...il.com>,
	moderated for non-subscribers <linux-arm-kernel@...ts.infradead.org>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev, stable@...r.kernel.org,
	Matthew Maurer <mmaurer@...gle.com>, Ralf Jung <post@...fj.de>,
	Jubilee Young <workingjubilee@...il.com>
Subject: Re: [PATCH] arm64: rust: clean Rust 1.85.0 warning using softfloat
 target

On Mon, Feb 10, 2025 at 05:37:32PM +0100, Miguel Ojeda wrote:
> Starting with Rust 1.85.0 (to be released 2025-02-20), `rustc` warns
> [1] about disabling neon in the aarch64 hardfloat target:
> 
>     warning: target feature `neon` cannot be toggled with
>              `-Ctarget-feature`: unsound on hard-float targets
>              because it changes float ABI
>       |
>       = note: this was previously accepted by the compiler but
>               is being phased out; it will become a hard error
>               in a future release!
>       = note: for more information, see issue #116344
>               <https://github.com/rust-lang/rust/issues/116344>
> 
> Thus, instead, use the softfloat target instead.
> 
> While trying it out, I found that the kernel sanitizers were not enabled
> for that built-in target [2]. Upstream Rust agreed to backport
> the enablement for the current beta so that it is ready for
> the 1.85.0 release [3] -- thanks!
> 
> However, that still means that before Rust 1.85.0, we cannot switch
> since sanitizers could be in use. Thus conditionally do so.
> 
> Cc: <stable@...r.kernel.org> # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Matthew Maurer <mmaurer@...gle.com>
> Cc: Alice Ryhl <aliceryhl@...gle.com>
> Cc: Ralf Jung <post@...fj.de>
> Cc: Jubilee Young <workingjubilee@...il.com>
> Link: https://github.com/rust-lang/rust/pull/133417 [1]
> Link: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/arm64.20neon.20.60-Ctarget-feature.60.20warning/near/495358442 [2]
> Link: https://github.com/rust-lang/rust/pull/135905 [3]
> Link: https://github.com/rust-lang/rust/issues/116344
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> ---
> Matthew: if you could please give this a go and confirm that it is fine
> for Android (with sanitizers enabled, for 1.84.1 and 1.85.0), then a
> Tested-by tag would be great. Thanks!

Patch looks fine to me, but I'll wait for Matthew to confirm that it
works for them. I'm also fine with adding the rustc-min-version helper
at the same time, tbh -- it's not exactly rocket science, but it would
need an Ack from Masahiro.

Will