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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1f2ad273-f3a5-4d16-95a4-b8d960410917@app.fastmail.com>
Date: Thu, 05 Dec 2024 12:58:22 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Andy Shevchenko" <andy@...nel.org>
Cc: "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "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 12:17, Andy Shevchenko wrote:
> On Thu, Dec 05, 2024 at 11:09:41AM +0100, Arnd Bergmann wrote:
>> 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,
>
> But does this affect building, e.g., for Quark on my Skylake desktop?

Not at the moment:

- the bug I'm fixing in the patch at hand is currently only present
  when building 64-bit kernels

- For a 64-bit target such as a Pineview Atom, it's only a problem
  if the toolchain default is -arch=native and you build with
  CONFIG_GENERIC_CPU

- If we add support for configuring -march=native and you build
  using that option on a Skylake host, that would be equally
  broken for 32-bit Quark or 64-bit Pineview targets that are
  lacking some of the instructions present in Skylake.

As I said earlier, I don't think we should offer the 'native'
option for 32-bit targets at all. For 64-bit, we either decide
it's a user error to enable -march=native, or change it to
-mtune=native to avoid the problem.

     Arnd