Re: [Qemu-devel] [PATCH] Fix -kernel on target-ppc

[Aurelien Jarno <aurelien@aurel32.net>]

On Sun, Jan 25, 2009 at 01:28:36AM +0100, Alexander Graf wrote:
>
>
>
>
> On 25.01.2009, at 00:59, Aurelien Jarno <aurelien@aurel32.net> wrote:
>
>> On Sat, Jan 24, 2009 at 10:57:19PM +0100, Alexander Graf wrote:
>>>
>>> On 24.01.2009, at 22:48, Aurelien Jarno wrote:
>>>
>>>> On Sat, Jan 24, 2009 at 09:58:35PM +0100, Alexander Graf wrote:
>>>>> OpenBIOS searches for the preloaded kernel at 0x1000000, so let's
>>>>> put it there instead of an invalid location.
>>>>
>>>> Your patch is actually wrong, the second argument of load_elf() is  
>>>> an
>>>> offset to the physical address (as found in the elf header), and  
>>>> not a
>>>> load address.
>>>>
>>>> By chance the physical address of a >= 2.6.25 kernel is  
>>>> 0x00000000, so
>>>> your patch works. But it will break supports for < 2.6.25 kernel as
>>>> their physical address is 0xc0000000. Not that they are only the
>>>> default
>>>> values, they can be changed in the .config file.
>>>
>>> Aah, that explains why :-).
>>>
>>>> I have already proposed a patch to use the virtual address of the  
>>>> elf
>>>> header as done by yaboot or quik, but I have been told it is  
>>>> actually
>>>> wrong.
>>>>
>>>> We have to find another way to load the elf file at a fixed address.
>>>
>>> Hm - can't we just give load_elf an override value for the base  
>>> address?
>>>
>>>            /* address_offset is hack for kernel images that are
>>>               linked at the wrong physical address.  */
>>>            addr = ph->p_paddr + address_offset;
>>>
>>>            cpu_physical_memory_write_rom(addr, data, mem_size);
>>>
>>> Just pass another variable here that overrides addr and doesn't
>>> calculate it?
>>
>> Except that they can be more than one segment to load, so the last one
>> will overwrite the previous ones. The PowerPC kernels I have seen only
>> have one load segment, but I am not sure it is always the case.
>
> But then the addr hack wouldn't work either, right? It's just a question 
> if addr_offset is relative or absolute here.

addr_offset is just an offset that is added to the load address of the
elf header.

> And fwiw in this case relative to the elf header's value doesn't make  
> any sense at all when the firmware expects the blob on a specific  
> address.

As far as I understand it has been done like that to be able to support
multiple segments. If the elf header says that the first segment has to 
be loaded at 0xc0000000 and the second at 0xd0000000, loading both at
0x10000000 won't work. Loading them with an offset of -0xb000000 will
load the first one at 0x10000000 and the second one just after at
0x20000000.

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net