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

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Message-Id: <74e8e9c6-8205-413a-97a4-aae32042c019@app.fastmail.com>
Date: Thu, 05 Dec 2024 11:09:41 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Andy Shevchenko" <andy@...nel.org>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc: "Arnd Bergmann" <arnd@...nel.org>, linux-kernel@...r.kernel.org,
 x86@...nel.org, "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>, "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 Thu, Dec 5, 2024, at 09:13, Andy Shevchenko wrote:
> On Wed, Dec 04, 2024 at 03:33:19PM -0800, Linus Torvalds wrote:
>> On Wed, 4 Dec 2024 at 11:44, Arnd Bergmann <arnd@...db.de> wrote:
>
> ...
>
>> Will that work when you cross-compile? No. Do we care? Also no. It's
>> basically a simple "you want to optimize for your own local machine"
>> switch.
>
> Maybe it's okay for 64-bit machines, but for cross-compiling for 32-bit on
> 64-bit. I dunno what '-march=native -m32' (or equivalent) will give in such
> cases.

>From the compiler's perspective this is nothing special, it just
builds a 32-bit binary that can use any instruction supported in
32-bit mode of that 64-bit CPU, the same as the 32-bit CONFIG_MCORE2
option that I disallow in patch 04/11.

     Arnd