linux-kernel - Re: [PATCH 1/2] Documentation: clarify PR_SPEC_L1D

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Message-ID: <z5zeptd2yij7e435u4xgdqsvnqf6hwjkwixajlh3u4nggp6gej@ej743d4adbb3>
Date: Wed, 15 Oct 2025 16:42:42 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Brendan Jackman <jackmanb@...gle.com>, 
	Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Balbir Singh <sblbir@...zon.com>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Documentation: clarify PR_SPEC_L1D_FLUSH

On Wed, Oct 15, 2025 at 02:41:18PM -0700, Kees Cook wrote:
> On Wed, Oct 15, 2025 at 05:02:05PM +0000, Brendan Jackman wrote:
> > For PR_SPEC_STORE_BYPASS and PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE
> > means "disable the speculation bug" i.e. "enable the mitigation".
> > 
> > For PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE means "disable the mitigation".
> > This is not obvious, so document it.
> 
> The only thing I can find in Debian Code Search that actually uses
> PR_SPEC_L1D_FLUSH is stress-ng, and it literally just toggles it before
> restoring it:
> https://sources.debian.org/src/stress-ng/0.19.05-1/stress-prctl.c?hl=893#L893
> 
> I wonder if we should just fix the prctl to match the existing
> behaviors?

This feature has existed for almost 5 years, I don't think we can just
reverse the functionality like that?  There could be proprietary users
out there (e.g., cloud companies).

-- 
Josh