linux-kernel - Re: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <20251022095657.GBaPiqaYxPMonQWLtw@fat_crate.local>
Date: Wed, 22 Oct 2025 11:56:57 +0200
From: Borislav Petkov <bp@...en8.de>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Jonathan Corbet <corbet@....net>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Kirill A . Shutemov" <kas@...nel.org>, Xin Li <xin@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Sean Christopherson <seanjc@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	David Laight <david.laight.linux@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Kees Cook <kees@...nel.org>, Tony Luck <tony.luck@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: Re: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Tue, Oct 21, 2025 at 01:55:51PM -0700, Sohil Mehta wrote:
> In the series, we directly write to the CR4 bits, so they don't have any
> wrappers. But in the future, lass_enable()/lass_disable() could be
> confusing if wrappers were added for the CR4 toggling.

Are you envisioning to export the CR4.LASS toggling to users like those two or
is former going to be done only at those two places?

Because CR4 toggling is expensive so you probably don't want to do that very
often.

> There is an issue here which you had originally objected to.
> https://lore.kernel.org/lkml/20240710171836.GGZo7CbFJeZwLCZUAt@fat_crate.local/
> https://lore.kernel.org/lkml/20240711012333.GAZo80FU30_x77otP4@fat_crate.local/
> 
> These new versions of lass_disable()/lass_enable() will toggle the AC
> flag on older platforms without X86_FEATURE_LASS. It definitely makes
> the code easier to read and maintain if we are okay with the minor
> performance penalty.

Hmm, we probably should measure that. The text poking should be a relatively
seldom operation but we should at least do a quick measurement to see whether
something registers on the radar...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette