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

Kaplan, David
2025-10-27T14:19:36Z

[AMD Official Use Only - AMD Internal Distribution Only]

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> Sent: Monday, October 27, 2025 6:35 AM
> To: Kaplan, David <David.Kaplan@amd.com>; Juergen Gross <jgross@suse.com>;
> Thomas Gleixner <tglx@linutronix.de>; Borislav Petkov <bp@alien8.de>; Peter
> Zijlstra <peterz@infradead.org>; Josh Poimboeuf <jpoimboe@kernel.org>; Pawan
> Gupta <pawan.kumar.gupta@linux.intel.com>; Ingo Molnar <mingo@redhat.com>;
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> <hpa@zytor.com>
> Cc: Alexander Graf <graf@amazon.com>; Boris Ostrovsky
> <boris.ostrovsky@oracle.com>; linux-kernel@vger.kernel.org
> Subject: Re: [RFC PATCH 34/56] x86/alternative: Save old bytes for alternatives
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> On 10/15/25 16:45, Kaplan, David wrote:
> > [AMD Official Use Only - AMD Internal Distribution Only]
> >
> >> -----Original Message-----
> >> From: Juergen Gross <jgross@suse.com>
> >> Sent: Wednesday, October 15, 2025 5:38 AM
> >> To: Kaplan, David <David.Kaplan@amd.com>; Thomas Gleixner
> >> <tglx@linutronix.de>; Borislav Petkov <bp@alien8.de>; Peter Zijlstra
> >> <peterz@infradead.org>; Josh Poimboeuf <jpoimboe@kernel.org>; Pawan
> Gupta
> >> <pawan.kumar.gupta@linux.intel.com>; Ingo Molnar <mingo@redhat.com>;
> Dave
> >> Hansen <dave.hansen@linux.intel.com>; x86@kernel.org; H . Peter Anvin
> >> <hpa@zytor.com>
> >> Cc: Alexander Graf <graf@amazon.com>; Boris Ostrovsky
> >> <boris.ostrovsky@oracle.com>; linux-kernel@vger.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@amd.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