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

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Message-ID: <20251016142347.GV4067720@noisy.programming.kicks-ass.net>
Date: Thu, 16 Oct 2025 16:23:47 +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 02:16:04PM +0000, Kaplan, David wrote:

> > Yeah, makes sense. But I like I wrote, I think prefix stuffing might be
> > a better option.
> 
> Ok, and that's because we need at most 3 prefixes as Intel uarch's
> don't have significant penalties at 3 prefixes, only at 4+?

Yeah, IIRC 3 was the magic number. Sadly there isn't much public
information on this. The best summary on the subject I could find was
here:

  https://reviews.llvm.org/D75945

> I'll need to check on this on the AMD side too.

The above, quoting Agner's optimization guide, says Bulldozer is
affected.