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

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Message-ID: <20251016110855.GO1386988@noisy.programming.kicks-ass.net>
Date: Thu, 16 Oct 2025 13:08:55 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: David Kaplan <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,
	"H . Peter Anvin" <hpa@...or.com>, Alexander Graf <graf@...zon.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 31/56] x86/alternative: Prepend nops with retpolines

On Thu, Oct 16, 2025 at 12:32:33PM +0200, Peter Zijlstra wrote:

> Alas, some micro-archs have significant decode penalties for >3
> prefixes, and filling 6 bytes will need 4 prefixes :-(

N/m this, because if it has 6 bytes, then the indirect call will need 3
bytes and we're back to needing 3 prefixes.