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

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Date:	Mon, 21 Apr 2008 11:11:11 -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)

* Mathieu Desnoyers (compudj@...stal.dyndns.org) wrote:
> * 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
> >
> 
> 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

err, by thread flag, I meant thread preempt count. And it's not in
registers, so it has to be read from the data cache (it's clearly
already there).

> 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.
> 
> 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

-- 
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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