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

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Message-ID:
 <LV3PR12MB9265E559972E1C6693AAE7E794FCA@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Mon, 27 Oct 2025 14:19:33 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Nikolay Borisov <nik.borisov@...e.com>, 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: Nikolay Borisov <nik.borisov@...e.com>
> Sent: Monday, October 27, 2025 6:35 AM
> To: Kaplan, David <David.Kaplan@....com>; 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; 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
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On 10/15/25 16:45, Kaplan, David wrote:
> > [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.
>
> It's not just about memory usage per-se but also memory pressure from
> allocation and the resulting fragmentation, though I'd think that
> majority of the allocation will fit into kmalloc-32 bucket, still having
> them as part of the kernel image eliminates the additional allocs.

I see.  Just to understand, the issue is more with the numerous small allocations right? (that is the kmalloc at each alt_site)  And less about the single large allocation of the array?

I'm just thinking about the retpoline_site handling too.  That one also has a large dynamically allocated array, although it does not have numerous small allocations because the size of each instruction is constrained to at most 6 bytes.

Thanks
--David Kaplan