linux-kernel - Re: [PATCH 10/19] x86/dumpstack: add get_stack

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Message-ID: <20160726164751.44xcnegpw2x3v2o6@treble>
Date:	Tue, 26 Jul 2016 11:47:51 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface

On Mon, Jul 25, 2016 at 05:09:44PM -0700, Andy Lutomirski wrote:
> On Sat, Jul 23, 2016 at 7:04 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > Am I correct in understanding that there can only be one level of NMI
> > nesting at any given time?  If so, could we make it easier on the
> > unwinder by putting the nested NMI on a separate software stack, so the
> > "next stack" pointers are always in the same place?  Or am I just being
> > naive?
> 
> I think you're being naive :)

Another dumb question: since NMIs are reentrant, have you considered
removing the NMI IST entry, and instead just have NMIs keep using the
current stack?

The first NMI could then be switched to an NMI software stack, like IRQs
(assuming there's a way to do that atomically!).  And then determining
the context of subsequent NMIs would be straightforward, and we'd no
longer need to jump through all those horrible hoops in the entry code
to deal with NMI nesting.

Now you can tell me what else I'm missing...

-- 
Josh