linux-kernel - Re: [PATCH 2/5] x86/entry/64, entry: Set up a valid sysenter stack and prepare for 32-bit merge

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Date:	Thu, 23 Jul 2015 12:10:01 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Willy Tarreau <w@....eu>, Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/5] x86/entry/64, entry: Set up a valid sysenter stack
 and prepare for 32-bit merge

On Thu, 23 Jul 2015 09:04:38 -0700
Andy Lutomirski <luto@...capital.net> wrote:

> On Thu, Jul 23, 2015 at 9:01 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Thu, 23 Jul 2015 08:31:40 -0700
> > Andy Lutomirski <luto@...nel.org> wrote:
> >
> >> Oddly, 64-bit kernels already allocate a percpu sysenter stack, but
> >> they don't enable it.  Enable the stack and tweak the rest of the
> >> sysenter setup code to be similar to the 32-bit version.
> >
> > I'm curious. Did you do any investigation into the 64bit code to why it
> > wasn't set? Do you know if that was just overlooked with some of the
> > merging between i386 and x86_64 systems?
> >
> > I'm not asking you to do it if you have not, but if you have, I think
> > it would be more comforting to know that it was just overlooked than
> > there being some other subtle reason.
> 
> I think it's just that it was never necessary.  #DB has always used
> IST on x86_64 (IIRC it was like that in the very first commit), so the
> sysenter stack was never actually referenced.
> 

Hmm, should this be commented? If it is never referenced, I could
imagine that would add some confusion to future reviewers of this code.

-- Steve
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