Re: [PATCH v3 01/12] x86, boot: move verify_cpu.S after 0x200

[H. Peter Anvin]

On 11/21/2012 11:45 AM, Yinghai Lu wrote:
> On Wed, Nov 21, 2012 at 9:23 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 11/20/2012 11:15 PM, Yinghai Lu wrote:
>>>
>>> We are short of space before 0x200 that is entry for startup_64.
>>>
>>> And we can not change startup_64 to other value --- ABI ?
>>
>>
>> Here you are saying "I don't understand how this works."  It is YOUR
>> responsibility to find out and write a definite statement rather than
>> leaving that to the reader, or expect the maintainer to edit this.
> 
> actually, i can not find that out.
> in the code of arch/x86/boot/compressed/head_64.S
> 
>         /*
>          * Be careful here startup_64 needs to be at a predictable
>          * address so I can export it in an ELF header.  Bootloaders
>          * should look at the ELF header to find this address, as
>          * it may change in the future.
>          */
>         .code64
>         .org 0x200
> ENTRY(startup_64)
>         /*
>          * We come here either from startup_32 or directly from a
>          * 64bit bootloader.  If we come here from a bootloader we depend on
>          * an identity mapped page table being provied that maps our
>          * entire text+data+bss and hopefully all of memory.
>          */
> #ifdef CONFIG_EFI_STUB
>         /*
>          * The entry point for the PE/COFF executable is 0x210, so only
>          * legacy boot loaders will execute this jmp.
>          */
>         jmp     preferred_addr
> 
>         .org 0x210
>         mov     %rcx, %rdi
> 
> and it says that 0x200 will be changed later..
> 
> so you said it has to stay with 0x200, do you mean 0x210 from PE/COFF
> force that?
> 
> wonder if you are considering attatched patch to move startup_64 down...
> we could kill one jmp.
> 

The comment is just plain wrong.  It assumes you're loading an ELF file,
whereas in practice that is rarely true.

This does explain why the poor ABI, though.  A jump table at the
beginning would have been a lot cleaner.

	-hpa


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