linux-kernel - Re: [PATCH RFC] x86: Add CONFIG_KERNEL

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Message-ID: <Z7tqFCo4smCysrfQ@gmail.com>
Date: Sun, 23 Feb 2025 19:33:56 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Jann Horn <jannh@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>, linux-efi@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] x86: Add CONFIG_KERNEL_UNCOMPRESSED support


* Ard Biesheuvel <ardb@...nel.org> wrote:

> > But I wasn't sure how to wire that up in a nice way. I guess the 
> > nicest option would be to create a separate kconfig variable for 
> > the compression level to use for any cmd_lz4/cmd_lz4_with_size 
> > invocations in the build process; and then maybe only make this 
> > option visible if LZ4 is selected as kernel compression method?
> >
> > Another option would be to create a new option in the "Kernel 
> > compression mode" choice menu with a name like "LZ4 (fast)", turn 
> > CONFIG_KERNEL_LZ4 into an internal flag that is selected by both 
> > LZ4 variants shown in the choice menu, and duplicate some of the 
> > make rules, but that seems overly complicated.
> >
> 
> I didn't realise that KERNEL_UNCOMPRESSED already exists and you are 
> just wiring it up for x86. But I still think that we should avoid 
> that, not only because it is yet another bzImage format but also 
> because I still see a 3x size reduction even with the fastest 
> setting.
> 
> I think adding one Kconfig symbol that depends on KERNEL_LZ4 and 
> switches from -9 to -1 for LZ4 only is reasonable.

Maybe a CONFIG_COMPRESS_FAST option that maps to and enables the 
current fastest compressor? Then we can also add LZ4_FAST and map it to 
it. (And if a future fastest compressor is added it can change this 
mapping.) Or something like that?

And if there's still a measurable difference in total build time (say 
larger than 1%) from doing it all uncompressed, then I think we should 
go with Jann's original patch that hooks up zero-compression on x86. It 
doesn't look overly complicated.

Thanks,

	Ingo