linux-kernel - Re: [PATCH v5 2/7] x86/mm: Use a single cache hot per-CPU variable to record pgdir

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Message-ID: <awmpxjln22i5zmnv3wcwhzvpbbjqmhiw3onmpq66owbtdoujs5@f336cwpvlasn>
Date: Tue, 20 May 2025 14:03:20 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Brian Gerst <brgerst@...il.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v5 2/7] x86/mm: Use a single cache hot per-CPU variable
 to record pgdir_shift

On Tue, May 20, 2025 at 12:41:41PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Shrink the global variable 'pgdir_shift' to a single byte, and move it
> to the cache hot per-CPU variable section so that it can be accessed
> virtually for free.

Hm. I am not convinced about per-CPU being useful here. Read-only cache
lines can be shared between cores anyway.

Putting it to __ro_after_init should do the trick, no?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov