linux-kernel - Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL

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Message-ID: <20251102200902.GDaQe6Xo0plI8glTbE@fat_crate.local>
Date: Sun, 2 Nov 2025 21:11:15 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Kaplan, David" <David.Kaplan@....com>
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>,
	Klaus Kusche <klaus.kusche@...puterix.info>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL_MSG

On Fri, Oct 10, 2025 at 01:14:37AM +0200, Borislav Petkov wrote:
> On Thu, Oct 09, 2025 at 06:13:20PM +0000, Kaplan, David wrote:
> > If we really want to optimize a CONFIG_CPU_MITIGATIONS=n kernel, why not
> > also look at alternatives that will never be enabled?
> > 
> > All the extra NOPs I think can affect performance in various ways and if
> > there was a way at compile time to know that the alternative would never be
> > enabled (because the feature is completely dependent on a cpu mitigation)
> > you could eliminate the NOPs...
> 
> I don't see why not. That mitigations gunk should be just like any other
> CONFIG_-controllable item and not simply "leak" into the build because, oh
> well, we've done it this way from the very beginning and no one should touch
> it.
> 
> So yeah, eventually we should try this. I've been experimenting with
> untangling bugs.c today, will check how much it actually saves us when
> I finish beating the stinking pile into submission.

So I can't say that it is insignificant:

   text    data     bss     dec     hex filename
30787172        11613226        7863812 50264210        2fef892 vmlinux.with
30210273        11598338        6061260 47869871        2da6faf vmlinux.without

that's ~500K code on my tailored config, meaning I haven't even enabled all
mitigations. So I'm thinking we should do this, but slowly, piecemeal, until
we have settled on the layout.

I'll send some patches soon to gather opinions...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette