linux-kernel - Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang

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Date:   Thu, 16 Sep 2021 11:42:19 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com, llvm@...ts.linux.dev,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for
 clang

On 9/16/2021 10:18 AM, Borislav Petkov wrote:
> On Mon, Aug 23, 2021 at 07:26:39PM -0700, Nathan Chancellor wrote:
> 
> A couple of nitpicks:
> 
>> Subject: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang
> 
> Make that prefix into "x86/build: "

Done, I'll be sure to keep that prefix in mind for future flag-based 
changes.

>> clang does not support -falign-jumps and only recently gained support
>> for -falign-loops. When one of the configuration options that adds these
>> flags is enabled, clang warns and all cc-{disable-warning,option} that
>> follow fail because -Werror gets added to test for the presence of this
>> warning:
>>
>> clang-14: warning: optimization flag '-falign-jumps=0' is not supported
>> [-Wignored-optimization-argument]
>>
>> To resolve this, add a couple of cc-option calls when building with
>> clang; gcc has supported these options since 3.2 so there is no point in
>> testing for their support. -falign-functions was implemented in clang-7,
>> -falign-loops was implemented in clang-14, and -falign-jumps has not
>> been implemented yet.
>>
>> Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/
> 
> Also, there should be a second Link: tag which points to this mail
> thread so that we can find it later, when we dig for the "why we did
> that" question :)
> 
> I.e.,
> 
> Link: 20210824022640.2170859-2-nathan@...nel.org

Sure thing, kind of hard to do that on the initial submission but I will 
do it for the v2 shortly :)

>> Reported-by: kernel test robot <lkp@...el.com>
>> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
>> ---
>>   arch/x86/Makefile_32.cpu | 12 +++++++++---
>>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> with that:
> 
> Acked-by: Borislav Petkov <bp@...e.de>

Thank you for the ack. The conflicting changes that I mentioned in the 
cover letter have been merged in 5.15-rc1 so if you guys want to take 
these changes via -tip, just holler for an ack from Masahiro on the 
second patch on v2 (but I am going with the assumption this will be 
merged via the kbuild tree).

Cheers,
Nathan