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

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Message-ID: <20080421154756.GA2424@Krystal>
Date:	Mon, 21 Apr 2008 11:47:56 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Pavel Machek <pavel@....cz>, 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)

* 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)?
>
> 	-hpa
>

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).

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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