linux-kernel - Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

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Message-ID: <20150410231831.GF2366@two.firstfloor.org>
Date:	Sat, 11 Apr 2015 01:18:31 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andi Kleen <andi@...stfloor.org>, Andi Kleen <ak@...ux.intel.com>,
	X86 ML <x86@...nel.org>, Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>, Jan Beulich <JBeulich@...e.com>
Subject: Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

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@...stfloor.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@...ux.intel.com -- Speaking for myself only.
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