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Date:   Mon, 29 Apr 2019 17:45:04 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
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, Apr 29, 2019 at 05:08:46PM -0700, Sean Christopherson wrote:
> On Mon, Apr 29, 2019 at 03:22:09PM -0700, Linus Torvalds wrote:
> > On Mon, Apr 29, 2019 at 3:08 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > FWIW, Lakemont (Quark) doesn't block NMI/SMI in the STI shadow, but I'm
> > > not sure that counters the "horrible errata" statement ;-).  SMI+RSM saves
> > > and restores STI blocking in that case, but AFAICT NMI has no such
> > > protection and will effectively break the shadow on its IRET.
> > 
> > Ugh. I can't say I care deeply about Quark (ie never seemed to go
> > anywhere), but it's odd. I thought it was based on a Pentium core (or
> > i486+?). Are you saying those didn't do it either?
> 
> 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.