linux-kernel - Re: [PATCH 09/11] x86: rework CONFIG_GENERIC

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Message-ID: <6c037258-4263-426d-beb2-e6a0697be3ab@mailbox.org>
Date: Wed, 4 Dec 2024 16:36:40 +0100
From: Tor Vic <torvic9@...lbox.org>
To: Arnd Bergmann <arnd@...nel.org>, linux-kernel@...r.kernel.org,
 x86@...nel.org
Cc: Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andy Shevchenko <andy@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Davide Ciminaghi <ciminaghi@...dd.com>, Paolo Bonzini <pbonzini@...hat.com>,
 kvm@...r.kernel.org
Subject: Re: [PATCH 09/11] x86: rework CONFIG_GENERIC_CPU compiler flags

On 12/4/24 11:30, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Building an x86-64 kernel with CONFIG_GENERIC_CPU is documented to
> run on all CPUs, but the Makefile does not actually pass an -march=
> argument, instead relying on the default that was used to configure
> the toolchain.
> 
> In many cases, gcc will be configured to -march=x86-64 or -march=k8
> for maximum compatibility, but in other cases a distribution default
> may be either raised to a more recent ISA, or set to -march=native
> to build for the CPU used for compilation. This still works in the
> case of building a custom kernel for the local machine.
> 
> The point where it breaks down is building a kernel for another
> machine that is older the the default target. Changing the default
> to -march=x86-64 would make it work reliable, but possibly produce
> worse code on distros that intentionally default to a newer ISA.
> 
> To allow reliably building a kernel for either the oldest x86-64
> CPUs or a more recent level, add three separate options for
> v1, v2 and v3 of the architecture as defined by gcc and clang
> and make them all turn on CONFIG_GENERIC_CPU. Based on this it
> should be possible to change runtime feature detection into
> build-time detection for things like cmpxchg16b, or possibly
> gate features that are only available on older architectures.
> 

Hi Arnd,

Similar but not identical changes have been proposed in the past several 
times like e.g. in 1, 2 and likely even more often.

Your solution seems to be much cleaner, I like it.

That said, on my Skylake platform, there is no difference between 
-march=x86-64 and -march=x86-64-v3 in terms of kernel binary size or 
performance.
I think Boris also said that these settings make no real difference on 
code generation.

Other settings might make a small difference (numbers are from 2023):
   -generic:       85.089.784 bytes
   -core2:         85.139.932 bytes
   -march=skylake: 85.017.808 bytes

----
[1] 
https://lore.kernel.org/all/4_u6ZNYPbaK36xkLt8ApRhiRTyWp_-NExHCH_tTFO_fanDglEmcbfowmiB505heI4md2AuR9hS-VSkf4s90sXb5--AnNTOwvPaTmcgzRYSY=@proton.me/

[2] 
https://lore.kernel.org/all/20230707105601.133221-1-dimitri.ledkov@canonical.com/