linux-kernel - Re: [PATCH v4 10/13] x86/mm: Simplify clear_page

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Message-ID: <20250616143800.GE1613376@noisy.programming.kicks-ass.net>
Date: Mon, 16 Jun 2025 16:38:00 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
	mingo@...hat.com, mjguzik@...il.com, luto@...nel.org,
	acme@...nel.org, namhyung@...nel.org, tglx@...utronix.de,
	willy@...radead.org, jon.grimm@....com, bharata@....com,
	raghavendra.kt@....com, boris.ostrovsky@...cle.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH v4 10/13] x86/mm: Simplify clear_page_*

On Mon, Jun 16, 2025 at 07:35:48AM -0700, Dave Hansen wrote:
> On 6/15/25 22:22, Ankur Arora wrote:
> > clear_page_rep() and clear_page_erms() are wrappers around "REP; STOS"
> > variations. Inlining gets rid of the costly call/ret (for cases with
> > speculative execution related mitigations.)
> 
> Could you elaborate a bit on which "speculative execution related
> mitigations" are so costly with these direct calls?

Pretty much everything with RETHUNK set I would imagine.