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

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Message-ID: <754ee3ec-8819-47df-8251-dec2906a020b@suse.com>
Date: Mon, 27 Oct 2025 13:34:30 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
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" <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



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.
> 
> Thanks
> --David Kaplan