linux-kernel - RE: [RFC PATCH 34/56] x86/alternative: Save old bytes for alternatives

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Message-ID:
 <LV3PR12MB9265295ABF26898CA59B86D594E8A@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Wed, 15 Oct 2025 13:45:42 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Juergen Gross <jgross@...e.com>, Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 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>
CC: 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 34/56] x86/alternative: Save old bytes for
 alternatives

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Juergen Gross <jgross@...e.com>
> Sent: Wednesday, October 15, 2025 5:38 AM
> To: Kaplan, David <David.Kaplan@....com>; Thomas Gleixner
> <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Peter Zijlstra
> <peterz@...radead.org>; 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; H . Peter Anvin
> <hpa@...or.com>
> Cc: Alexander Graf <graf@...zon.com>; Boris Ostrovsky
> <boris.ostrovsky@...cle.com>; linux-kernel@...r.kernel.org
> Subject: Re: [RFC PATCH 34/56] x86/alternative: Save old bytes for alternatives
>
> On 13.10.25 16:34, David Kaplan wrote:
> > Save the existing instruction bytes at each alternative site when patching.
> > This is only done the first time, and these will be used later to help
> > restore the code back to its original form.
> >
> > Signed-off-by: David Kaplan <david.kaplan@....com>
>
> Instead of saving the original instructions at runtime, why don't you
> expand struct alt_instr to have an additional offset to a saved copy
> of the original instruction, located in .altinstr_replacement?
>
> The new field should be guarded with #ifdef CONFIG_DYNAMIC_MITIGATIONS,
> of course, like the added handling in the ALTERNATIVE() macros.
>

That's an interesting idea, I think that could work.  That would make the kernel image on disk (slightly) larger though, as the original bytes will essentially be duplicated (at the original location and in .altinstr_replacement).  I'm not sure which is the better trade-off (kernel image bytes on disk vs runtime memory usage).  Although I think we're talking about a relatively small amount of memory regardless.  Most of the runtime overhead of dynamic mitigations comes from remembering the call sites/returns.

Thanks
--David Kaplan