linux-kernel - Re: [tip: x86/core] x86/cpufeatures: Generate the <asm/cpufeaturemasks.h> header based on build config

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Message-ID: <CAHk-=whChrRQ0kuRuZo2+9D+J2BO9Q=1CV2eEk_yjrNi8kdjBg@mail.gmail.com>
Date: Thu, 20 Mar 2025 09:56:25 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Michael Kelley <mhklinux@...look.com>
Cc: "H. Peter Anvin (Intel)" <hpa@...or.com>, "Xin Li (Intel)" <xin@...or.com>, "Borislav Petkov (AMD)" <bp@...en8.de>, 
	Ingo Molnar <mingo@...nel.org>, Nikolay Borisov <nik.borisov@...e.com>, 
	"x86@...nel.org" <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [tip: x86/core] x86/cpufeatures: Generate the <asm/cpufeaturemasks.h>
 header based on build config

On Thu, 20 Mar 2025 at 09:23, Michael Kelley <mhklinux@...look.com> wrote:
>
> With this commit, changing the modified time on .config causes cpufeaturemasks.h
> to be rebuilt, and then all dependencies on cpufeaturemasks.h are also rebuilt. It
> doesn't figure out that nothing has changed in .config. A fair amount of
> unnecessary rebuilding is done.

Hmm. Normally it is the tools that *create* the config file that
should notice that it hasn't changed, and not write a new one.

See conf_write() in scripts/kconfig/confdata.c for the particular case
of the regular "make *config*'

> I'm seeing this changed behavior specifically with the make-kpkg script on
> Ubuntu 20.04. It's not clear why make-kpkg is updating the modified time
> on .config, but prior to this commit, that update had no negative effect.

It sounds like the make-kpkg script should probably be fixed to not
overwrite the config file unconditionally, and do that "make
oldconfig" and friends do, which is to notice when it hasn't changed
and leave it alone.

                   Linus