Re: [tip: x86/core] x86/alternative: Patch a single alternative location only once

["Jürgen Groß" <jgross@suse.com>]

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On 14.10.25 16:12, Borislav Petkov wrote:
> On Tue, Oct 14, 2025 at 04:08:47PM +0200, Jürgen Groß wrote:
>> And just for the record: NMI handling is using ALTERNATIVE_2, so you
>> can't just "disable interrupts" and be sure it will work.
> 
> Are you fixing anything you're actually hitting while patching alternatives or
> are you talking hypothetically here?

I really ran into issues while writing my paravirt MSR series, as I had
an ALTERNATIVE3() invocation modifying the original instruction (the
indirect paravirt call) with an WRMSR and then trying to turn the no
longer existing indirect call into a direct one.

So I didn't have the intermediate issue in interrupt handling, but the
issue with rules how to place indirect call replacements in ALTERNATIVEn()
invocations.

The interrupt handling still might be a thing, while NMI is for sure rather
unlikely.

> 
> Because I can't think of an easy way to trigger NMIs that early during boot,
> when not even SMP is up.
> 

NMIs are probably really a more theoretical issue. And even if NMIs are
affected, they will still be affected in case they happen just when half of
the new instruction bytes have been written.


Juergen

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