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

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Date:	Mon, 21 Apr 2008 10:51:25 -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:
> Pavel Machek wrote:
>> On Thu 2008-04-17 16:14:10, Mathieu Desnoyers wrote:
>>> (hopefully finally CCing LKML) :)
>>>
>>> Implements an alternative iret with popf and return so trap and exception
>>> handlers can return to the NMI handler without issuing iret. iret would 
>>> cause
>>> NMIs to be reenabled prematurely. x86_32 uses popf and far return. x86_64 
>>> has to
>>> copy the return instruction pointer to the top of the previous stack, 
>>> issue a
>>> popf, loads the previous esp and issue a near return (ret).
>> sounds expensive. Does it slow down normal loads?
>
> It should *only* be used to return from NMI, #MC or INT3 (breakpoint), 
> which should never happen in normal operation, and even then only when 
> interrupting another NMI or #MC handler.
>
> 	-hpa
>

Sorry Pavel, for some reason you message did not reach my inbox. hpa is
right : this code path is only taken to return to the NMI handler from
a trap or exception or, possibly, machine check exception.

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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