Date: Mon, 8 Sep 2003 15:05:43 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Pavel Machek <pavel@suse.cz>
Cc: Mikael Pettersson <mikpe@csd.uu.se>, jim.houston@comcast.net,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Pentium Pro - sysenter - doublefault
Message-ID: <20030908140543.GA26269@mail.jlokier.co.uk>
References: <1061498486.3072.308.camel@new.localdomain> <16197.14968.235907.128727@gargle.gargle.HOWL> <20030825060900.GA21213@mail.jlokier.co.uk> <20030903125019.GN1358@openzaurus.ucw.cz>
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Pavel Machek wrote:
> > "SEP is unsupported".  It's interesting that Pentium Pro erratum #82
> > is "SYSENTER/SYSEXIT instructions can implicitly load 'null segment
> > selector' to SS and CS registers", implying that SYSENTER does
> > _something_ useful on PPros.
> 
> Well, with CS==0 machine is not going to survive too long.
> If it only happens sometimes you might catch the double fault
> and fixup, but....

The erratum only applies when you load CS==0 _deliberately_, by setting
the MSR to that.

I'm wondering what happens when you don't do silly things - what is
the undocumented behaviour of SYSENTER/SYSEXIT on those chips?

I vaguely recall reading details about the behaviour change made by
Intel, around the time it was done, but I can't see to find it
anywhere.

-- Jamie
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