linux-kernel - Re: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi

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Message-ID: <20251016185241.GG3289052@noisy.programming.kicks-ass.net>
Date: Thu, 16 Oct 2025 20:52:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Kaplan, David" <David.Kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, Alexander Graf <graf@...zon.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()

On Thu, Oct 16, 2025 at 03:34:14PM +0000, Kaplan, David wrote:

> I will note that on AMD, MFENCE has the required serializing
> properties (like SERIALIZE).  On AMD, we could use MFENCE in
> sync_core() which is probably faster that iret_to_self().  But
> again...do we really care.

About faster, no. But MFENCE has the benefit of not causing VMEXITs and
also not being IRET, so I'm not opposed to you using that as an AMD
version of SERIALIZE for the time being.