On Monday 21 August 2006 15:29, Magnus Damm wrote:
> On 8/21/06, Andi Kleen <[email protected]> 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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