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Date: Sat, 11 Apr 2015 01:18:31 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andi Kleen <andi@firstfloor.org>, Andi Kleen <ak@linux.intel.com>,
        X86 ML <x86@kernel.org>, Andrew Lutomirski <luto@kernel.org>,
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        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        Jan Beulich <JBeulich@suse.com>
Subject: Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base
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On Fri, Apr 10, 2015 at 04:15:39PM -0700, Andy Lutomirski wrote:
> On Fri, Apr 10, 2015 at 4:05 PM, Andi Kleen <andi@firstfloor.org> wrote:
> >> One might argue that this code serves no purpose, but it's there, so
> >> we had better keep our per-invocation usage of DEBUG_STACK within 4k.
> >
> > Only if you run NKLD. I doubt KDB or GDB support nesting.
> > We can ask Jan if he still uses it.
> 
> You can trigger these things in various ways with kprobes,
> single-stepping, and user-space watchpoints.  These things can
> definitely nest in interesting ways.  Whether or not the IST shift
> mechanism is involved if they nest is a complicated question, because
> there's also the "debug" IDT in play.

Yes we need it for (old style) kprobes in those paths.

For user space it shouldn't matter.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.