linux-kernel - Re: [PATCH][RFC] x86_64: Reload CS when startup

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Date:	Mon, 21 Aug 2006 15:02:58 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Magnus Damm <magnus@...inux.co.jp>
Cc:	fastboot@...ts.osdl.org, linux-kernel@...r.kernel.org, ak@...e.de
Subject: Re: [PATCH][RFC] x86_64: Reload CS when startup_64 is used.

Magnus Damm <magnus@...inux.co.jp> writes:

> x86_64: Reload CS when startup_64 is used.
>
> The current x86_64 startup code never reloads CS during the early boot process
> if the 64-bit function startup_64 is used as entry point. The 32-bit entry 
> point startup_32 does the right thing and reloads CS, and this is what most 
> people are using if they use bzImage.
>
> This patch fixes the case when the Linux kernel is booted into using kexec
> under Xen. The Xen hypervisor is using large CS values which makes the x86_64
> kernel fail - but only if vmlinux is booted, bzImage works well because it
> is using the 32-bit entry point.
>
> The main question is if we require that the boot loader should setup CS
> to some certain offset to be able to boot the kernel. The sane solution IMO
> should be that the kernel requires that the loaded descriptors are correct, 
> but that the exact offset within the GDT the boot loader is using should not 
> matter. This is the way the i386 boot works if I understand things correctly.

What extra reload of cs does Xen introduce?

I'm not really comfortable with a half virtualized case.

Eric
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