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SubjectRe: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk and bzImage above 4G
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Err, lretq it is.

Borislav Petkov <bp@alien8.de> wrote:

>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.

--
Sent from my mobile phone. Please excuse brevity and lack of formatting.


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