Re: [PATCH] Use correct x86 reboot vector

From: ebiederm@xmission.com (Eric W. Biederman)
To: Matt Mackall <mpm@selenic.com>
Cc: Linus Torvalds <torvalds@transmeta.com>,
	Davide Libenzi <davidel@xmailserver.org>,
	Jamie Lokier <jamie@shareable.org>, Jos Hulzink <josh@stack.nl>,
	Andi Kleen <ak@muc.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Use correct x86 reboot vector
Date: 11 May 2003 13:16:26 -0600	[thread overview]
Message-ID: <m1of295m5x.fsf@frodo.biederman.org> (raw)
In-Reply-To: <20030511190023.GC9173@waste.org>

Matt Mackall <mpm@selenic.com> writes:

> There's a missing piece of behavior here that's probably fatal.
> Namely, the next time the CS descriptor is loaded, even with the same
> value, the high bits are lost. So, for example, if you're running BIOS
> out of ROM, decompressing it into the top of 20-bit address space,
> then long jumping to your uncompressed code, you don't want to find
> yourself back in ROM.
> 
> Perhaps there's a trick that can be played with loading the descriptor
> into CS and then clearing the descriptor table without flushing, but it
> sounds rather dubious..

If PE is really disabled that should bit should come for free.  And it
is why it is so hard to fake this behavior.

Eric