linux-kernel - Re: [RFC PATCH 31/56] x86/alternative: Prepend nops with retpolines

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Message-ID: <20251016140731.GS4067720@noisy.programming.kicks-ass.net>
Date: Thu, 16 Oct 2025 16:07:31 +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 31/56] x86/alternative: Prepend nops with retpolines

On Thu, Oct 16, 2025 at 01:27:53PM +0000, Kaplan, David wrote:

> > Ah, I found it, you freeze everything, which puts it at safe points.
> 
> Yes.  In fact, I think you were the one who pointed me in that direction :)

Heh, yeah, I remembered talking to you about this, but had forgotten all
details. I quickly checked the cover letter, but found insufficient
detail there, so I went and asked. Then later memory started coming back
and I went looking for it in later patches.

> Despite the freezer though, this patch is necessary in particular
> because stop_machine_nmi() uses an indirect branch to run the handler.
> Which means that while patching is going on, all cores are inside a
> function which is going to return to after the indirect call site.
> And so that needs to be the end of the 5 (or 6) byte sequence.

Yeah, makes sense. But I like I wrote, I think prefix stuffing might be
a better option.