linux-kernel - Re: [PATCH v3] x86/bugs: Add a separate config for each mitigation

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Date:   Thu, 5 Oct 2023 18:25:45 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Breno Leitao <leitao@...ian.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, leit@...a.com,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] x86/bugs: Add a separate config for each mitigation

+ Linus.

On Thu, Sep 28, 2023 at 05:45:32AM -0700, Breno Leitao wrote:
> On Wed, Jun 28, 2023 at 07:21:28AM -0700, leitao@...ian.org wrote:
> > From: Breno Leitao <leitao@...ian.org>
> > 
> > Create an entry for each CPU mitigation under
> > CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
> > them at compilation time.
> > 
> > If a mitigation is disabled at compilation time, it could be enabled at
> > runtime using kernel command line arguments.
> 
> I had a chat about this topic with Boris and Thomas at Kernel Recipes,
> and I would like to summarize the current state, and get it moving
> forward.
> 
> 1) The hardware mitigations are half-way added to KCONFIG. I.e., half of
> the hardware mitigations are specified under SPECULATION_MITIGATIONS,
> but not all of them.
> 	* You can enabled/disabled just half of them at build time.
> 
> 2) It is impossible to build a kernel with speculative mitigations
> disabled.
> 	* The only way to disable the mitigations is at boot time,
> 	  using the "mitigations=off" boot parameter.
> 
> 
> So, disabling SPECULATION_MITIGATIONS, will only disable the mitigations
> that are under SPECULATION_MITIGATIONS. Other mitigations will continue
> to be enabled by default. This is is misleading for the user.
> 
> Here are a few options moving forward:
> 
> 1) Create one Kconfig entry per mitigation, so, the user can pick and
> choose what to enable and disable. (Version 3 of this patch. May need a
> re-spin due to the new mitigations being added.)
> 
> 2) Keep the Kconfig entries as-is. Create a new Kconfig entry
> (CPU_MITIGATIONS_DEFAULT_OFF?) to disable the mitigations by default,
> similarly to the `mitigations=off` boot parameter (v1 of this patch)
> 
> 3) Same as 2, but, reusing SPECULATION_MITIGATIONS instead of
> creating a new Kconfig entry.
> 
> 4) Remove the current entries in SPECULATION_MITIGATIONS and the fine
> control on what to enable/disable?!
> 
> What is the preferred way?

I happen to know that Linus wanted those per mitigation, perhaps to be
able to disable only a subset of them.

Linus, what are you thoughts on it, should we continue with a Kconfig
option per mitigation or should we hide them all behind a single Kconfig
option - which would be a lot simpler and easier?

Apparently people want to completely remove the mitigations crap for
some configurations at build time already.

Thx.

-- 
Regards/Gruss,
    Boris.

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