linux-kernel - Re: New x86-Setup code breaks HVM-XEN boot

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Message-ID: <46DCF84F.1010302@zytor.com>
Date:	Tue, 04 Sep 2007 07:16:47 +0100
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christian Ehrhardt <lk@...e.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: New x86-Setup code breaks HVM-XEN boot

Christian Ehrhardt wrote:
> 
> I took the trouble to bisect (manually) exactly which change in the new
> boot code triggers this problem.
> 
> The problem is with the lgdt instruction. Apparently XEN does not keep
> the contents of the 48-bit gdt_48 data structure that is passed to lgdt
> in the XEN machine state. Instead it appears to save the _address_ of the
> 48-bit descriptor somewhere. Unfortunately this data happens to reside on the
> stack and is probably no longer availiable at the time of the actual
> protected mode jump.
> 
> This is most likely a XEN-bug but given that there is a on line patch
> to work around this problem, the linux kernel should probably do this.
> My fix is to make the gdt_48 description in setup_gdt static (in
> setup_idt this is already the case). This allows the kernel to boot under
> XEN-hvm again.

Would indeed be a Xen bug, and a pretty serious one too.  Quite frankly, 
it reflects some pretty fundamental misconceptions about how x86 works.

> 
> Sometimes it is a bit disappointing if quite some debuggin work results
> in a on line patch :-) Pleae consider applying.
> 

LOL, well, that's usually a good thing.

	-hpa

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