Messages in this thread | | | Subject | Re: [Fastboot] [PATCH][RFC] x86_64: Reload CS when startup_64 is used. | From | Magnus Damm <> | Date | Tue, 22 Aug 2006 09:47:08 +0900 |
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On Mon, 2006-08-21 at 16:16 +0200, Andi Kleen wrote: > 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.
I think reloading CS is the right thing to do. IMO it is not sane to depend on that the 64-bit boot loader sets up CS to 0x18 for us.
Having a dependency like that (unless there is a good reason and it is documented somehow) is good to avoid, regardless of 64-bit uncompress or not. I mean, if you plan on making the bzImage code 64-bit then it needs to reload CS too, right?
Thanks,
/ magnus
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