linux-kernel - [PATCH] arch: um: Pass the correct Rust target and options with gcc

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Message-ID: <20250210105353.2238769-2-davidgow@google.com>
Date: Mon, 10 Feb 2025 18:53:51 +0800
From: David Gow <davidgow@...gle.com>
To: "Thomas Weißschuh" <linux@...ssschuh.net>, Miguel Ojeda <ojeda@...nel.org>, 
	Richard Weinberger <richard@....at>, Johannes Berg <johannes@...solutions.net>, 
	Roberto Sassu <roberto.sassu@...weicloud.com>, 
	Vincenzo Palazzo <vincenzopalazzodev@...il.com>
Cc: David Gow <davidgow@...gle.com>, x86@...nel.org, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	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, llvm@...ts.linux.dev, 
	linux-um@...ts.infradead.org, Anton Ivanov <anton.ivanov@...bridgegreys.com>, 
	kunit-dev@...glegroups.com, linux-kernel@...r.kernel.org
Subject: [PATCH] arch: um: Pass the correct Rust target and options with gcc

In order to work around some issues with disabling SSE on older versions
of gcc (compilation would fail upon seeing a function declaration
containing a float, even if it was never called or defined), the
corresponding CFLAGS and RUSTFLAGS were only set when using clang.

However, this led to two problems:
- Newer gcc versions also wouldn't get the correct flags, despite not
  having the bug.
- The RUSTFLAGS for setting the rust target definition were not set,
  despite being unrelated. This works by chance for x86_64, as the
  built-in default target is close enough, but not for 32-bit x86.

Move the target definition outside the conditional block, and update the
condition to take into account the gcc version.

Fixes: a3046a618a28 ("um: Only disable SSE on clang to work around old GCC bugs")
Signed-off-by: David Gow <davidgow@...gle.com>
---

When combined with Thomas' patch [1], this gets Rust/UML building
correctly with gcc. (Indeed, Thomas' patch works fine by itself for the
64-bit case, and the 32-bit build is broken anyway due to the Rust block
driver not supporting 64-bit atomics on 32-bit systems.)

Given the other patches will probably go in via the rust-for-linux tree,
I'd be happy for this to do so as well (given it mostly affects Rust),
but it's not a disaster if they go in independently: there shouldn't be
any merge conflicts, and they work independently.

Note also that I don't actually have an old enough gcc nearby, so
haven't actually tested this on gcc < 11. So please let me know if this
breaks for you.

Cheers,
-- David

[1]: https://lore.kernel.org/rust-for-linux/20250208-rust-kunit-v1-1-94a026be6d72@weissschuh.net/

---
 arch/x86/Makefile.um | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
index a46b1397ad01..c86cbd9cbba3 100644
--- a/arch/x86/Makefile.um
+++ b/arch/x86/Makefile.um
@@ -7,12 +7,13 @@ core-y += arch/x86/crypto/
 # GCC versions < 11. See:
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
 #
-ifeq ($(CONFIG_CC_IS_CLANG),y)
-KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
-KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
+ifeq ($(call gcc-min-version, 110000)$(CONFIG_CC_IS_CLANG),y)
+KBUILD_CFLAGS +=  -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
 KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
 endif
 
+KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
+
 ifeq ($(CONFIG_X86_32),y)
 START := 0x8048000
 
-- 
2.48.1.502.g6dc24dfdaf-goog