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

Nikolay Borisov
2025-10-29T09:37:27Z

On 10/27/25 16:19, Kaplan, David wrote:
> [AMD Official Use Only - AMD Internal Distribution Only]
>
>> -----Original Message-----
>> From: Nikolay Borisov <nik.borisov@suse.com>
>> 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>;
>> 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
>>
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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?

Yep, do you have some statistics how many allocs have to be done?

>
> 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
>
>