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: <CAMj1kXFxRZWsML_5FZvZjwOPO8cvsAwDqvX1686bqqfqkD_PHg@mail.gmail.com>
Date: Tue, 20 May 2025 19:03:37 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>, Ard Biesheuvel <ardb+git@...gle.com>, 
	linux-kernel@...r.kernel.org, x86@...nel.org, Ingo Molnar <mingo@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v5 2/7] x86/mm: Use a single cache hot per-CPU variable to
 record pgdir_shift

On Tue, 20 May 2025 at 16:35, Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, May 20, 2025 at 01:28:52PM +0200, Ard Biesheuvel wrote:
> > Are you saying we shouldn't optimize prematurely? You must be new here :-)
>
> Right, but do you see __pgdir_shift on a seriously hot path to warrant this?
>

No. But if you had read the next couple of patches, you would have
noticed that PGDIR_SHIFT, PTRS_PER_P4D and pgtable_l5_enabled() will
all be derived from this variable, and the latter currently uses code
patching (in cpu_feature_enabled())

This is also explained in the cover letter btw