linux-kernel - Re: [PATCH] kbuild: Use -fmin-function-alignment when available

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Date: Tue, 13 Feb 2024 15:04:03 +0100
From: Petr Pavlu <petr.pavlu@...e.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: masahiroy@...nel.org, nicolas@...sle.eu, mark.rutland@....com,
 linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: Use -fmin-function-alignment when available

On 2/13/24 01:27, Nathan Chancellor wrote:
> Hi Petr,
> 
> On Mon, Feb 12, 2024 at 03:53:55PM +0100, Petr Pavlu wrote:
>> GCC recently added option -fmin-function-alignment, which should appear
>> in GCC 14. Unlike -falign-functions, this option causes all functions to
>> be aligned at the specified value, including the cold ones.
>>
>> Detect availability of -fmin-function-alignment and use it instead of
>> -falign-functions when present. Introduce CC_HAS_SANE_FUNCTION_ALIGNMENT
>> and make the workarounds for the broken function alignment conditional
>> on this setting.
>>
>> Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
>> ---
>>  Makefile                       |  7 ++++++-
>>  arch/Kconfig                   |  8 ++++++++
>>  include/linux/compiler_types.h | 10 +++++-----
>>  kernel/exit.c                  |  5 ++++-
>>  4 files changed, 23 insertions(+), 7 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 7e0b2ad98905..9516e43f6e45 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -974,7 +974,12 @@ export CC_FLAGS_CFI
>>  endif
>>  
>>  ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0)
>> -KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
>> +# Set the minimal function alignment. Try to use the newer GCC option
>> +# -fmin-function-alignment, or fall back to -falign-funtions. See also
>> +# CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT.
>> +KBUILD_CFLAGS += $(call cc-option, \
>> +	-fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT), \
>> +	-falign-functions=$(CONFIG_FUNCTION_ALIGNMENT))
>>  endif
>>  
>>  # arch Makefile may override CC so keep this after arch Makefile is included
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index a5af0edd3eb8..e2448f927fae 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -1507,4 +1507,12 @@ config FUNCTION_ALIGNMENT
>>  	default 4 if FUNCTION_ALIGNMENT_4B
>>  	default 0
>>  
>> +config CC_HAS_SANE_FUNCTION_ALIGNMENT
>> +	# Detect availability of the GCC option -fmin-function-alignment which
>> +	# guarantees minimal alignment for all functions. GCC 13 and older have
>> +	# only -falign-functions which the compiler ignores for cold functions
>> +	# and this hence requires extra care in the kernel. Clang provides
>> +	# strict alignment always when using -falign-functions.
>> +	def_bool $(cc-option, -fmin-function-alignment=8) || CC_IS_CLANG
>> +
> 
> I think this configuration should be split into something like
> CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT that has the cc-option check then
> CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT can depend on that configuration
> or Clang as you have it here, so that we can drop the cc-option check in
> the main Makefile and have it be:
> 
> ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT
> KBUILD_CFLAGS += -fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT)
> else
> KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
> endif
> 
> It is wasteful to call cc-option twice in my opinion, especially if we
> are checking it in Kconfig.

Makes sense, thanks.

-- Petr