linux-kernel - [tip: x86/kconfig] x86/kbuild/64: Restrict clang versions that can use '-march=native'

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Message-ID: <174368720008.30396.5633938489882625239.tip-bot2@tip-bot2>
Date: Thu, 03 Apr 2025 13:33:20 -0000
From: "tip-bot2 for Nathan Chancellor" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Nathan Chancellor <nathan@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Tor Vic <torvic9@...lbox.org>,
 Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>,
 Juergen Gross <jgross@...e.com>, "H. Peter Anvin" <hpa@...or.com>,
 Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/kconfig] x86/kbuild/64: Restrict clang versions that can
 use '-march=native'

The following commit has been merged into the x86/kconfig branch of tip:

Commit-ID:     ad9b861824ac55d81431815fffaaff5e981badc1
Gitweb:        https://git.kernel.org/tip/ad9b861824ac55d81431815fffaaff5e981badc1
Author:        Nathan Chancellor <nathan@...nel.org>
AuthorDate:    Mon, 24 Mar 2025 20:23:00 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 25 Mar 2025 08:24:06 +01:00

x86/kbuild/64: Restrict clang versions that can use '-march=native'

There are two issues that can appear when building with clang and
'-march=native'.

The first issue is a compiler crash in the ipv6 stack with clang-18,
such as when building allmodconfig. This was frequently reported on the
LLVM issue tracker:

  # Link: https://github.com/llvm/llvm-project/issues?q=is%3Aissue%20ip6_rcv_core

  Stack dump:
  0.      Program arguments: clang ... -march=native ... net/ipv6/ip6_input.c
  1.      <eof> parser at end of file
  2.      Code generation
  3.      Running pass 'Function Pass Manager' on module 'net/ipv6/ip6_input.c'.
  4.      Running pass 'X86 DAG->DAG Instruction Selection' on function '@..._rcv_core'

The second issue is certain -Wframe-larger-than warnings that appear
with clang versions prior to 19, which introduced an optimization that
produces much better code:

  # Link: https://github.com/llvm/llvm-project/commit/90ba33099cbb17e7c159e9ebc5a512037db99d6d [2]

  clang-18:

    drivers/media/pci/saa7164/saa7164-core.c:605:20: error: stack frame size (2392) exceeds limit (2048) in 'saa7164_irq' [-Werror,-Wframe-larger-than]
      605 | static irqreturn_t saa7164_irq(int irq, void *dev_id)
          |                    ^

  clang-19:

    drivers/media/pci/saa7164/saa7164-core.c:605:20: error: stack frame size (216) exceeds limit (128) in 'saa7164_irq' [-Werror,-Wframe-larger-than]
      605 | static irqreturn_t saa7164_irq(int irq, void *dev_id)
          |                    ^

Restrict the testing of the availability of '-march=native' to all
supported GCC versions and clang-19 and newer to avoid these known
resolved issues.

Fixes: 0480bc7e65dc ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Tor Vic <torvic9@...lbox.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20250324-x86-march-native-clang-19-and-newer-v1-1-3a05ed32a89e@kernel.org
---
 arch/x86/Kconfig.cpu | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index 87bede9..f928cf6 100644
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -248,6 +248,12 @@ endchoice
 config CC_HAS_MARCH_NATIVE
 	# This flag might not be available in cross-compilers:
 	def_bool $(cc-option, -march=native)
+	# LLVM 18 has an easily triggered internal compiler error in core
+	# networking code with '-march=native' on certain systems:
+	# https://github.com/llvm/llvm-project/issues/72026
+	# LLVM 19 introduces an optimization that resolves some high stack
+	# usage warnings that only appear wth '-march=native'.
+	depends on CC_IS_GCC || CLANG_VERSION >= 190100

 config X86_NATIVE_CPU
 	bool "Build and optimize for local/native CPU"