Re: [PATCH 2/2] x86, Makefile: Add new generic x86-64 settings v2/v3/v4

From: torvic9@mailbox.org
To: Borislav Petkov <bp@alien8.de>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"clang-built-linux@googlegroups.com" 
	<clang-built-linux@googlegroups.com>,
	"graysky@archlinux.us" <graysky@archlinux.us>,
	"masahiroy@kernel.org" <masahiroy@kernel.org>,
	Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 2/2] x86, Makefile: Add new generic x86-64 settings v2/v3/v4
Date: Wed, 18 Aug 2021 17:08:40 +0200 (CEST)	[thread overview]
Message-ID: <1708009975.124311.1629299321026@office.mailbox.org> (raw)
In-Reply-To: <YRwBs89eogLJEiRn@zn.tnic>

> Borislav Petkov <bp@alien8.de> hat am 17.08.2021 20:36 geschrieben:
> 
>  
> On Tue, Aug 17, 2021 at 04:24:48PM +0200, torvic9@mailbox.org wrote:
> > Add new generic x86-64 CPU tunings introduced with recent versions of
> > gcc and clang, as documented in x86-64-psABI [1].
> > 
> > This is taken straight from graysky's CPU optimization patch with minor
> > modifications [2].
> > 
> > [1] https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/77566eb03bc6a326811cb7e9a6b9396884b67c7c
> > [2] https://github.com/graysky2/kernel_compiler_patch
> > 
> > Signed-off-by: Tor Vic <torvic9@mailbox.org>
> > ---
> >  arch/x86/Kconfig.cpu  | 26 +++++++++++++++++++++++++-
> >  arch/x86/Makefile.cpu |  6 ++++++
> >  2 files changed, 31 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
> > index 814fe0d349b0..a2c872aa5a0b 100644
> > --- a/arch/x86/Kconfig.cpu
> > +++ b/arch/x86/Kconfig.cpu
> > @@ -294,6 +294,30 @@ config GENERIC_CPU
> >  	  Generic x86-64 CPU.
> >  	  Run equally well on all x86-64 CPUs.
> >  
> > +config GENERIC_CPU_V2
> > +	bool "Generic-x86-64-v2"
> > +	depends on ( CC_IS_GCC && GCC_VERSION > 110000 ) || ( CC_IS_CLANG && CLANG_VERSION >= 120000 )
> > +	depends on X86_64
> > +	help
> > +	  Generic x86-64 CPU.
> > +	  Run equally well on all x86-64 CPUs with min support of x86-64-v2.
> > +
> > +config GENERIC_CPU_V3
> > +	bool "Generic-x86-64-v3"
> > +	depends on ( CC_IS_GCC && GCC_VERSION > 110000 ) || ( CC_IS_CLANG && CLANG_VERSION >= 120000 )
> > +	depends on X86_64
> > +	help
> > +	  Generic x86-64-v3 CPU with v3 instructions.
> > +	  Run equally well on all x86-64 CPUs with min support of x86-64-v3.
> > +
> > +config GENERIC_CPU_V4
> > +	bool "Generic-x86-64-v4"
> > +	depends on ( CC_IS_GCC && GCC_VERSION > 110000 ) || ( CC_IS_CLANG && CLANG_VERSION >= 120000 )
> > +	depends on X86_64
> > +	help
> > +	  Generic x86-64 CPU with v4 instructions.
> > +	  Run equally well on all x86-64 CPUs with min support of x86-64-v4.
> 
> Every once in a while, patches like that pop up without any real numbers
> advocating for the additional and perhaps unnecessary complexity.
> 
> If those -march switches don't bring any measureable improvements - and
> I highly doubt they do - you shouldn't waste time with submitting them.

Borislav,
I ran some quick checks and benchmarks, and your doubts seem to be justified.
A 5.14-rc6 kernel compiled with the default 'generic' and one built with
'x86-64-v3' even have the exact same uncompressed file size.
Benchmarks were inconclusive as well.
So I will only resubmit the first patch of this series, with the changes
suggested by Masahiro and Christoph.

> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette