linux-kernel - Re: [PATCH 07/19] x86/dumpstack: add IRQ_USABLE_STACK

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Message-ID: <20160722014812.3adsrej7zj4qpbff@treble>
Date:	Thu, 21 Jul 2016 20:48:12 -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 07/19] x86/dumpstack: add IRQ_USABLE_STACK_SIZE define

On Thu, Jul 21, 2016 at 03:01:10PM -0700, Andy Lutomirski wrote:
> On Thu, Jul 21, 2016 at 2:21 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > For reasons unknown, the x86_64 irq stack starts at an offset 64 bytes
> > from the end of the page.  At least make that explicit.
> 
> This is a change in behavior -- see below.  Please mention this in the
> changelog.

Ah, right.

> 
> >
> > FIXME: Can we just remove the 64 byte gap?  If not, at least document
> > why.
> 
> I have no clue.
> 
> >
> >         irq_stack_end   = (unsigned long *)(per_cpu(irq_stack_ptr, cpu));
> > -       irq_stack       = (unsigned long *)(per_cpu(irq_stack_ptr, cpu) - IRQ_STACK_SIZE);
> > +       irq_stack       = (unsigned long *)(per_cpu(irq_stack_ptr, cpu) -
> > +                         IRQ_USABLE_STACK_SIZE);
> 
> This is different.

If nobody knows the reason for it, I may just remove it.  It doesn't
seem to blow anything up on my system.  I tried digging through the git
history but it's been there since the beginning of git time.

-- 
Josh