From: Andi Kleen <ak@suse.de>
To: "Magnus Damm" <magnus.damm@gmail.com>
Subject: Re: [Fastboot] [PATCH][RFC] x86_64: Reload CS when startup_64 is used.
Date: Mon, 21 Aug 2006 16:16:50 +0200
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Cc: "Magnus Damm" <magnus@valinux.co.jp>, fastboot@lists.osdl.org,
       linux-kernel@vger.kernel.org, ebiederm@xmission.com
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On Monday 21 August 2006 15:29, Magnus Damm wrote:
> On 8/21/06, Andi Kleen <ak@suse.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.

Yes, that code always runs in the identity mapping and at 2MB.

> 
> What do you think about reloading CS? Is it the right thing to do, or
> is it correct as it is today where we depend on that CS == _KERNEL_CS?
> I need to fix kexec-tools regardless, but maybe it is a good idea to
> make the 64-bit kernel boot a bit robust too.

Reloading CS is ok, although longer term I plan to switch the kernel
to uncompress already in 64bit. Then you would need the same GDT anyways.

-Andi


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