linux-kernel - Re: [PATCH] fix i486 boot failure due to stale %ds

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Message-ID: <p733avl392r.fsf@bingen.suse.de>
Date:	Mon, 05 Nov 2007 00:05:48 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
	tglx@...utronix.de
Subject: Re: [PATCH] fix i486 boot failure due to stale %ds

Mikael Pettersson <mikpe@...uu.se> writes:

> Maybe not. I had a look in Intel's SDM Vol3, and the
> section "switching to protected mode" specifies that
> a move to %cr0 that sets PE should immediately be
> followed by a far jmp or call. They write that "random
> failures can occur if other instructions exist between
> [the move to %cr0] and [the far jmp/call]". The current
> version of pmjump.S does exactly that: it executes
> a bunch of moves to segment registers in that window.

iirc various Intel VT hypervisor implementations also have trouble
with not following this restriction. At least in the early Xen
days it was a frequent reason for boot failure.

-Andi
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