linux-kernel - RE: [RFC PATCH 01/56] Documentation/admin-guide: Add documentation

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Message-ID:
 <LV3PR12MB92652DD7F888DE1AC839F76194F6A@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Fri, 17 Oct 2025 14:04:10 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.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 01/56] Documentation/admin-guide: Add documentation

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> Sent: Thursday, October 16, 2025 4:25 PM
> To: Kaplan, David <David.Kaplan@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Peter
> Zijlstra <peterz@...radead.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 01/56] Documentation/admin-guide: Add documentation
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Mon, Oct 13, 2025 at 09:33:49AM -0500, David Kaplan wrote:
> > +Runtime Limitations
> > +^^^^^^^^^^^^^^^^^^^
> > +
> > +There are a few mitigations that cannot be toggled at runtime due to the way
> > +they are structured.  Specifically, kernel PTI (page table isolation) cannot be
> > +toggled because of the complexity of this mitigation.  Additionally, SMT cannot
> > +be disabled at runtime.  Therefore, if a bug mitigation requires disabling SMT,
> > +a warning message will be printed.
>
> Is there a particular reason SMT can't be disabled?  There's definitely
> a way to do it, see /sys/devices/system/cpu/smt/{active,control}.
>

Reason was I didn't realize you could do that :)

The existing bugs.c logic used cpu_smt_disable() which is an __init function.  But now I see that if you have CONFIG_HOTPLUG_SMT then cpuhp_smt_enable()/cpuhp_smt_disable() look like they should be able to handle this.

I will look into supporting this, if CONFIG_HOTPLUG_SMT is enabled.

Thanks
--David Kaplan