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Date:   Tue, 30 Apr 2019 13:17:40 +0200 (CEST)
From:   Jiri Kosina <jikos@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip
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On Mon, 29 Apr 2019, Linus Torvalds wrote:

> > > It's 486 based, but either way I suspect the answer is "yes".  IIRC,
> > > Knights Corner, a.k.a. Larrabee, also had funkiness around SMM and that
> > > was based on P54C, though I'm struggling to recall exactly what the
> > > Larrabee weirdness was.
> >
> > Aha!  Found an ancient comment that explicitly states P5 does not block
> > NMI/SMI in the STI shadow, while P6 does block NMI/SMI.
> 
> Ok, so the STI shadow really wouldn't be reliable on those machines. Scary.
> 
> Of course, the good news is that hopefully nobody has them any more, and 
> if they do, they presumably don't use fancy NMI profiling etc, so any 
> actual NMI's are probably relegated purely to largely rare and 
> effectively fatal errors anyway (ie memory parity errors).

FWIW, if that thing has local apic (I have no idea, I've never seen Quark 
myself), then NMIs are used to trigger all-cpu backtrace as well. Which 
still can be done in situations where the kernel is then expected to 
continue running undisrupted (softlockup, sysrq, hung task detector, ...).

Nothing to really worry about in the particular case of this HW perhaps, 
sure.

-- 
Jiri Kosina
SUSE Labs