Re: [PATCH v2 1/3] arm64: rust: Enable Rust support for AArch64

From: Boqun Feng <boqun.feng@gmail.com>
To: Jamie Cunliffe <Jamie.Cunliffe@arm.com>
Cc: catalin.marinas@arm.com, lina@asahilina.net,
	linux-arm-kernel@lists.infradead.org, ojeda@kernel.org,
	rust-for-linux@vger.kernel.org, steve.capper@arm.com,
	will@kernel.org
Subject: Re: [PATCH v2 1/3] arm64: rust: Enable Rust support for AArch64
Date: Tue, 15 Aug 2023 11:00:06 -0700	[thread overview]
Message-ID: <ZNu9JhYlsFWmliQE@boqun-archlinux> (raw)
In-Reply-To: <ZJ3fhb8QYdXhGl1r@boqun-archlinux>

On Thu, Jun 29, 2023 at 12:46:13PM -0700, Boqun Feng wrote:
> On Mon, Jun 12, 2023 at 04:26:39PM +0100, Jamie Cunliffe wrote:
> > > I don't think you need "-fp-armv8" here, because "aarch64-unknown-none"
> > > target doesn't have the target-feature "fp-arm64":
> > > 
> > > 	rustc +1.68.2 --print cfg --target aarch64-unknown-none | grep target_feature
> > > 
> > > 	target_feature="neon"
> > > 
> > > , and having the "-fp-armv8" gave me a warning at compile time:
> > > 
> > > 	  RUSTC L rust/core.o warning: unknown feature specified for `-Ctarget-feature`: `fp-armv8`
> > > 	  |
> > > 	  = note: it is still passed through to the codegen backend
> > > 	  = help: consider filing a feature request
> > 
> > Rust removed the fp feature for arm64 and tied it with Neon. However
> > this initially had a bug that meant it couldn't be disabled. This has
> > now been fixed in https://github.com/rust-lang/rust/pull/107294 so
> > once we can update rustc we can fix that warning by removing the
> > `-fp-armv8`. Until we do that we still need the `-fp-armv8` otherwise
> > we will get some floating point usage. This can be seen by doing an
> > objdump of core.o if you build after removing the `-fp-armv8`.
> > 
> 
> Thanks for the information (and the fix)! That's good to know. Looks
> like the fix will be in 1.71, then agree, we should keep `-fp-armv8`
> until we update to 1.71 or later.
> 

Just FYI, rustc has been bumped to 1.71:

	https://lore.kernel.org/rust-for-linux/CANiq72mrzALO3J2VvDntscXwt7Z9KzT4+S8sc_HGc8u8izjyLw@mail.gmail.com/

BTW, recently I learned that ARCH is set as "x86" by default even on
x86_64 host, so things like:

	+ifeq ($(ARCH),x86_64)
	+$(obj)/core.o: scripts/target.json
	+endif

"ifeq .." will return false if compiled with "make LLVM=1" even on a
x86_64 host. Maybe use

	ifdef CONFIG_X86_64

?

Regards,
Boqun

> Regards,
> Boqun
> 
> > 
> > You can also see that fp-armv8 is enabled in the aarch64-unknown-none target
> > here: https://raw.githubusercontent.com/rust-lang/rust/master/compiler/rustc_target/src/spec/aarch64_unknown_none.rs
> > 
> > 
> > Thanks,
> > Jamie.