linux-kernel - Re: [PATCH -tip 2/2] x86/hweight: Use POPCNT when available with X86_NATIVE

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Message-ID: <20250330191544.213e07d8@pumpkin>
Date: Sun, 30 Mar 2025 19:15:44 +0100
From: David Laight <david.laight.linux@...il.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip 2/2] x86/hweight: Use POPCNT when available with
 X86_NATIVE_CPU option

On Sun, 30 Mar 2025 18:07:30 +0200
Uros Bizjak <ubizjak@...il.com> wrote:

...
> The kernel is currently compiled with -mno-red-zone, gcc-15 introduces
> special "redzone" clobber to disable red-zone in the function that
> includes asm() when/if the kernel starts using redzone.

I really don't understand the point of a stack 'red-zone'.
In any function that contains memory writes the cost of a
'sub $n,%rsp' at the top and corresponding add at the bottom
is surely noise.

	David.