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

Kaplan, David
2025-10-30T14:39:17Z

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: David Laight <david.laight.linux@gmail.com>
> Sent: Wednesday, October 29, 2025 5:14 PM
> To: Kaplan, David <David.Kaplan@amd.com>
> Cc: Nikolay Borisov <nik.borisov@suse.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>; 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
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
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>
> On Wed, 29 Oct 2025 16:26:58 +0000
> "Kaplan, David" <David.Kaplan@amd.com> wrote:
>
> > [AMD Official Use Only - AMD Internal Distribution Only]
> >
> > > -----Original Message-----
> > > From: Nikolay Borisov <nik.borisov@suse.com>
> > > Sent: Wednesday, October 29, 2025 4:37 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
> > >
> > > Caution: This message originated from an External Source. Use proper caution
> > > when opening attachments, clicking links, or responding.
> > >
> > >
> > > 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
> > > >>
> > > >> 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@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?
> > >
> >
> > On a typical kernel, I'm seeing 6427 kmallocs() from this with a total size of
> ~36kb.
> >
> > If that is too many, another option could be to go through and figure out the total
> size needed and then do one big allocation.
>
> Is there also an 8 byte pointer to each allocation? They add up as well.
> Is may be worth doing multiple (say) 4k allocations in a list (or array of pointers).
> Then the pointer can be replaced by an offset into the overall 'big buffer'.
> Align the entries (a bit) and maybe a the 8 byte pointer can be replaced with
> a 16bit index?
>

Yes, there is an 8B pointer to each allocation (although I didn't include that in the number above).

There's a number of ways to optimize this, doing a single 'big buffer' with perhaps a 32-bit index seems rather straightforward. And maybe there are then further ways to squeeze this. But I think we're really talking about a small amount of memory, especially compared to the other overhead noted above.

Thanks
--David Kaplan