linux-kernel - Re: 4.4-rc5 Setting trap flag inside nmi handler results in HARD LOCKUP

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Message-ID: <CAO6TR8Vv5T81JbSLPKgVm=hj_f83b=iEfPLUbesRJ_6qDOr0oQ@mail.gmail.com>
Date:	Wed, 16 Dec 2015 11:42:11 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, peterz@...radead.org, luto@...nel.org
Subject: Re: 4.4-rc5 Setting trap flag inside nmi handler results in HARD LOCKUP

On 12/16/15, Jeff Merkey <linux.mdb@...il.com> wrote:
> Setting the (trap flag | resume flag) inside of an nmi handler results
> in a hard lockup while setting the resume flag works fine.
>
> The watchdog detector fails to detect the lockup.  I am currently
> examining the trap gate and interrupt gate setup on Linux and if
> anyone has any ideas it would be nice to be able to debug and step
> through the nmi handlers.  I got breakpoints to work.  I noticed
> kgdb/kdb just punts here and refuses to allow someone to step inside
> an nmi handler.
>
> There is no reason Linux should not allow this to work since windows
> does and every other OS out there.  I have seen this across some rex64
> sysret calls as well this lockup behavior.
>
> Anyone who is an intel expert with any clues would love some input if
> you know about this problem.
>
> Jeff
>

More info.  Linux is getting a trap and it looks like the IDT is
getting swapped when it gets it -- POW - Dead Linux.

Damn ...  Well, it will be long night and lots of builds ...

Jeff
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