Re: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk and bzImage above 4G

[Borislav Petkov <bp@alien8.de>]

On Sun, Dec 23, 2012 at 10:00:26AM -0800, Yinghai Lu wrote:
> On Sun, Dec 23, 2012 at 6:33 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> > Explanation please?
>
> you have following change in the patch
> 
>         /* Finally jump to run C code and to be on real kernel address
>          * Since we are running on identity-mapped space we have to jump
>          * to the full 64bit address, this is only possible as indirect
>          * jump.  In addition we need to ensure %cs is set so we make this
> -        * a far return.
> +        * a far jump.
>          */
> -       movq    initial_code(%rip),%rax
>         pushq   $0              # fake return address to stop unwinder
> -       pushq   $__KERNEL_CS    # set correct cs
> -       pushq   %rax            # target address in negative space
> -       lretq
> +       /* gas 2.22 is buggy and mis-assembles ljmpq */
> +       rex64 ljmp *initial_code(%rip)
> 
>  #ifdef CONFIG_HOTPLUG_CPU
>  /*
> 
> remove that change, AMD systems works again.

Right, the original code did a RET FAR by popping CS and rIP from the
stack. And we did prepare the stack properly before that so it worked.

Now, the ljmp translates to a JMP FAR:

ffffffff8100016e:       48 ff 2d ab a5 7f 00    rex.W ljmpq *0x7fa5ab(%rip)        # ffffffff817fa720 <initial_code>
ffffffff81000175:       66 66 2e 0f 1f 84 00    data32 nopw %cs:0x0(%rax,%rax,1)
ffffffff8100017c:       00 00 00 00 

and in 64-bit mode it has for an operand a 16-bit selector followed by a
32-bit offset.

Now, Intel SDM says also this:

REX.W + FF /5 JMP m16:64 A Valid N.E. Jump far, absolute indirect,
address given in m16:64.

And I don't think AMD supports a 64-bit offset. At least I don't see it
in the APM where it has only:

JMP FAR mem16:16 FF /5 Far jump indirect, with the target specified by a far
                      pointer in memory.
JMP FAR mem16:32 FF /5 Far jump indirect, with the target specified by a far
pointer in memory.

This is at least what I can see at a quick scan. I could ask around if
AMD actually supports that FF /5 with a REX.W prefix and it is not only
a documentation omission.

HTH.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--