From e58096244338308dde3fa76982c8042981ddda52 Mon Sep 17 00:00:00 2001 From: Nathan Chancellor Date: Tue, 20 Oct 2026 14:52:19 -0700 Subject: [PATCH] scripts/Kconfig.toolchain: Highlight cc-option differences between Kconfig and Kbuild Performing cc-option checks in Kconfig instead of Kbuild can help speed up incremental builds where the configuration does not change, as the results of the probes can be easily cached. However, there are two potential issues that folks need to consider when using cc-option in Kconfig compared to Kbuild. The first is that in Kbuild, cc-option additions are cumulative, so KBUILD_CFLAGS += $(call cc-option,-mfoo) KBUILD_CFLAGS += $(call cc-option,-mbar) results in '-mbar' being tested with '-mfoo' if it was supported. In Kconfig, '-mbar' would be tested without '-mfoo', which could change the result if '-mbar' depends on '-mfoo' being present, so it would need to test support with $(cc-option,-mbar -mfoo) The second is that the Kconfig stage is where questions like "is the target architecture 32-bit or 64-bit" or "is the target architecture big or little endian" are answered, which very much matters for testing compiler options. Toolchains may be configured for either option, so the kernel needs to pass compiler flags like '-m32', '-m64', '-mlittle-endian', and '-mbig-endian' to properly match a user's configuration choices. Without these flags, '-mfoo' is tested against a toolchain's configured default, resulting in an inconsistent answer to whether the flag is actually supported for the requested configuration. This could manifest as a build breakage from an unexpectedly included but unsupported flag or missing functionality from an unexpected omitted flag. Add a big comment summarizing these two issues to help future travelers. Link: https://patch.msgid.link/20261020-kconfig-fix-mfentry-check-v1-2-428258bc0fc0@kernel.org Signed-off-by: Nathan Chancellor --- scripts/Kconfig.toolchain | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/scripts/Kconfig.toolchain b/scripts/Kconfig.toolchain index 01b49fa5d6be..a99d875489ee 100644 --- a/scripts/Kconfig.toolchain +++ b/scripts/Kconfig.toolchain @@ -144,6 +144,25 @@ config CC_HAS_ASSUME config CC_HAS_NO_PROFILE_FN_ATTR def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror) +# Beware of two problems when using cc-option in Kconfig compared to Kbuild +# +# 1. cc-option in Kbuild tests the provided flags with all previously added +# CFLAGS, whereas cc-option in Kconfig only tests the provided flags with a +# small, constant set of additional flags. If a flag (e.g., '-mfoo') +# depends on another (e.g, '-fbar') to function properly, the dependency +# must be included in the cc-option call (e.g, '-fbar -mfoo'). +# +# 2. Bit and endianness flags ('-m32', '-m64', '-mbig-endian', and +# '-mlittle-endian') are not present in Kconfig cc-option checks because +# they may be included in KBUILD_CFLAGS / KBUILD_CPPFLAGS based on the +# finalized configuration result. This can result in inconsistent results +# when probing toolchains that support either option, as the toolchain's +# configured default tested in Kconfig may not support the tested flag but +# the post-configuration result in Kbuild would. +# +# In general, prefer adding truly generic compiler flags to avoid this +# pitfall. + config CC_OPT_ZERO_INIT_PADDING_BITS def_string "$(cc-option-str,-fzero-init-padding-bits=all)"