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

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Message-ID: <480CA337.3090709@zytor.com>
Date:	Mon, 21 Apr 2008 10:22:47 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Pavel Machek <pavel@....cz>
CC:	Mathieu Desnoyers <compudj@...stal.dyndns.org>, 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)

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

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