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

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Message-ID: <20060821141721.GA29498@in.ibm.com>
Date:	Mon, 21 Aug 2006 10:17:21 -0400
From:	Vivek Goyal <vgoyal@...ibm.com>
To:	Magnus Damm <magnus.damm@...il.com>
Cc:	Andi Kleen <ak@...e.de>, Magnus Damm <magnus@...inux.co.jp>,
	fastboot@...ts.osdl.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com
Subject: Re: [Fastboot] [PATCH][RFC] x86_64: Reload CS when startup_64 is used.

On Mon, Aug 21, 2006 at 10:29:36PM +0900, Magnus Damm wrote:
> On 8/21/06, Andi Kleen <ak@...e.de> wrote:
> >
> >>
> >> +     /* Reload CS with a value that is within our GDT. We need to do 
> >this
> >> +      * if we were loaded by a 64 bit bootloader that happened to use a
> >> +      * CS that is larger than the GDT limit. This is true if we came 
> >here
> >> +      * from kexec running under Xen.
> >> +      */
> >> +     movq    %rsp, %rdx
> >> +     movq    $__KERNEL_DS, %rax
> >> +     pushq   %rax /* SS */
> >> +     pushq   %rdx /* RSP */
> >> +     movq    $__KERNEL_CS, %rax
> >> +     movq    $cs_reloaded, %rdx
> >> +     pushq   %rax /* CS */
> >> +     pushq   %rdx /* RIP */
> >> +     lretq
> >
> >Can't you just use a normal far jump? That might be simpler.
> 
> I couldn't find a far jump that took a 64-bit address to jump to. But
> I guess that the kernel will be loaded in the lowest 4G regardless so
> I guess 32-bit pointers are ok, right? That will make it simpler for
> sure.
> 

Given the idea of relocatable kernel is floating around I would prefer if
we are not bounded by the restriction of loading a kernel in lowest 4G.

Thanks
Vivek
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