linux-kernel - Re: [RFC PATCH] x86 NMI-safe INT3 and Page Fault (v5)

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Message-ID: <20080421172327.GA32278@elf.ucw.cz>
Date:	Mon, 21 Apr 2008 19:23:28 +0200
From:	Pavel Machek <pavel@....cz>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, mingo@...e.hu, akpm@...l.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] x86 NMI-safe INT3 and Page Fault (v5)

On Mon 2008-04-21 11:47:56, Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@...or.com) wrote:
> > Mathieu Desnoyers wrote:
> >> Just to be clear : the added cost on normal interrupt return is to add a
> >> supplementary test of the thread flags already loaded in registers and
> >> a conditional branch. This is used to detect if we are nested over an
> >> NMI handler. I doubt anyone ever notice an impact caused by this added
> >> test/branch.
> >
> > Why the **** would you do this except in the handful of places where you 
> > actually *could* be nested over an NMI handler (basically #MC, #DB and 
> > INT3)?

> 
> There is also the page fault case. I think putting this test in
> ret_from_exception would be both safe (it is executed for any
> exception return) and fast (exceptions are rare).

Eh? I thought that page fault is one of the hottest paths in kernel
(along with syscall and packet receive/send)...
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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