linux-kernel - Re: [RFT PATCH v2 03/23] x86/boot: Drop global variables keeping track of LA57 state

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Message-ID: <CAHk-=wiaEzS_7CBVTz3RYnDt5zJus_GsPtfSjojkqiiMU-vSHQ@mail.gmail.com>
Date: Sun, 4 May 2025 07:58:24 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-efi@...r.kernel.org, x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Borislav Petkov <bp@...en8.de>, Dionna Amalie Glaze <dionnaglaze@...gle.com>, 
	Kevin Loughlin <kevinloughlin@...gle.com>, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFT PATCH v2 03/23] x86/boot: Drop global variables keeping
 track of LA57 state

On Sun, 4 May 2025 at 06:51, Ingo Molnar <mingo@...nel.org> wrote:
>
> Cannot pgtable_l5_enabled() be a single, simple percpu flag or so?

Isn't this logically something that should just be a static key? For
some reason I thought we did that already, but looking around, that
was only in the critical user access routines (and got replaced by the
'runtime-const' stuff)

But I guess that what Ard wants to get rid of is the variable itself,
and for early boot using static keys sounds like a bad idea.

Honestly, looking at this, I think we should fix the *users* of
pgtable_l5_enabled().

Because I think there are two distinct classes:

 - the stuff in <asm/pgtable.h> is exposed as the generic page table
accessor macros to "real" code, and should probably use a static key
(ie things like pgd_clear() / pgd_none() and friends)

 - but in code like __kernel_physical_mapping_init() feels to me like
using the value in %cr4 actually makes sense

but that looks like a much bigger and fundamental patch than Ard's.

                 Linus