boottime kernel relocation, what I missed?

boottime kernel relocation, what I missed?

[meerkat]

Good day all,

For the first time I begin working on PPC, and on low level, and right start
from boot sequence, one issue puzzled me.

After bootstrap code (zImage) uncompressed the kernel vmLinux to physical
memory (say from addr 0),
it jumps to the kernel entry point, _start, using physically address. 

 At this time, the MMU is not yet setup to map the kernel virtual address 
(which is statically linked against base address KERNELBASE) to the
physically address.

$ nm vmlinux |grep early_init
c038b8e0 T early_init


_start calls early_init before mmu is on to map the KERNEL_BASE to
physically address

The question is how "bl  early_init"  can branch to the early_init entry
point, properly, as early_init is still a virtual address?

Thanks

Jim
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Re: boottime kernel relocation, what I missed?

[meerkat]

Figure that out, the bootstrap actually mapped the first 16M from C000000 to
the physicall address,
so calling a c routine, as long as it is in the first 16M, is OK


meerkat wrote:
> 
> Good day all,
> 
> For the first time I begin working on PPC, and on low level, and right
> start from boot sequence, one issue puzzled me.
> 
> After bootstrap code (zImage) uncompressed the kernel vmLinux to physical
> memory (say from addr 0),
> it jumps to the kernel entry point, _start, using physically address. 
> 
>  At this time, the MMU is not yet setup to map the kernel virtual address 
> (which is statically linked against base address KERNELBASE) to the
> physically address.
> 
> $ nm vmlinux |grep early_init
> c038b8e0 T early_init
> 
> 
> _start calls early_init before mmu is on to map the KERNEL_BASE to
> physically address
> 
> The question is how "bl  early_init"  can branch to the early_init entry
> point, properly, as early_init is still a virtual address?
> 
> Thanks
> 
> Jim
> 

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Re: boottime kernel relocation, what I missed?

[Becky Bruce]

On Jul 13, 2007, at 5:07 PM, meerkat wrote:

>
> Figure that out, the bootstrap actually mapped the first 16M from  
> C000000 to
> the physicall address,
> so calling a c routine, as long as it is in the first 16M, is OK
>

I think you're still not understanding the fact that "bl" is a  
*relative* branch - the branch target in the instruction encoding is  
just an offset from the current address, not an effective address.   
The bl should work correctly whether the code is actually running at  
the link address reported by nm (0xcxxxxxxx in this case), or if it  
has been loaded and executed elsewhere.

Refer to the 32-bit Programming Environments Manual for PowerPC, or  
in the EREF (if you're using a BookE part - e500/e200) for more  
details.  If you're just learning PowerPC assembler, you should  
really give this book a good thourough read.

>
> meerkat wrote:
>>
>> Good day all,
>>
>> For the first time I begin working on PPC, and on low level, and  
>> right
>> start from boot sequence, one issue puzzled me.
>>
>> After bootstrap code (zImage) uncompressed the kernel vmLinux to  
>> physical
>> memory (say from addr 0),
>> it jumps to the kernel entry point, _start, using physically address.

If you read the book specified above, you will see that branch  
instructions always specify an effective address, not a physical  
address.  You can disable translation or map the address so EA=PA,  
but that's a different issue.

-Becky

Re: boottime kernel relocation, what I missed?

[meerkat]

Becky:

Thanks for the reply.   Agree I should read bookE, which given the short
schedule, it is still on my TODO list yet.

So are you saying linker does all the relative reference, that the bl  foo
is always relative even though the foo is defined in a different module? (In
my case, bl call is made in head.S, early_init() is defined in another file
setup.c).   

Regards,

Jim




Becky Bruce wrote:
> 
> 
> On Jul 13, 2007, at 5:07 PM, meerkat wrote:
> 
>>
>> Figure that out, the bootstrap actually mapped the first 16M from  
>> C000000 to
>> the physicall address,
>> so calling a c routine, as long as it is in the first 16M, is OK
>>
> 
> I think you're still not understanding the fact that "bl" is a  
> *relative* branch - the branch target in the instruction encoding is  
> just an offset from the current address, not an effective address.   
> The bl should work correctly whether the code is actually running at  
> the link address reported by nm (0xcxxxxxxx in this case), or if it  
> has been loaded and executed elsewhere.
> 
> Refer to the 32-bit Programming Environments Manual for PowerPC, or  
> in the EREF (if you're using a BookE part - e500/e200) for more  
> details.  If you're just learning PowerPC assembler, you should  
> really give this book a good thourough read.
> 
>>
>> meerkat wrote:
>>>
>>> Good day all,
>>>
>>> For the first time I begin working on PPC, and on low level, and  
>>> right
>>> start from boot sequence, one issue puzzled me.
>>>
>>> After bootstrap code (zImage) uncompressed the kernel vmLinux to  
>>> physical
>>> memory (say from addr 0),
>>> it jumps to the kernel entry point, _start, using physically address.
> 
> If you read the book specified above, you will see that branch  
> instructions always specify an effective address, not a physical  
> address.  You can disable translation or map the address so EA=PA,  
> but that's a different issue.
> 
> -Becky
> _______________________________________________
> Linuxppc-embedded mailing list
> Linuxppc-embedded@ozlabs.org
> https://ozlabs.org/mailman/listinfo/linuxppc-embedded
> 
> 

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Re: boottime kernel relocation, what I missed?

[Becky Bruce]

On Jul 16, 2007, at 2:36 PM, meerkat wrote:

> So are you saying linker does all the relative reference, that the  
> bl  foo
> is always relative even though the foo is defined in a different  
> module? (In
> my case, bl call is made in head.S, early_init() is defined in  
> another file
> setup.c).

If you see "bl foo", it's relative.  It has to be, because that's how  
the instruction works.   Also, it's not really in a different module,  
it's just a different file, but it's all linked into a single  
executable image.

Cheers,
-Becky

>
> Regards,
>
> Jim
>
>
>
>
> Becky Bruce wrote:
>>
>>
>> On Jul 13, 2007, at 5:07 PM, meerkat wrote:
>>
>>>
>>> Figure that out, the bootstrap actually mapped the first 16M from
>>> C000000 to
>>> the physicall address,
>>> so calling a c routine, as long as it is in the first 16M, is OK
>>>
>>
>> I think you're still not understanding the fact that "bl" is a
>> *relative* branch - the branch target in the instruction encoding is
>> just an offset from the current address, not an effective address.
>> The bl should work correctly whether the code is actually running at
>> the link address reported by nm (0xcxxxxxxx in this case), or if it
>> has been loaded and executed elsewhere.
>>
>> Refer to the 32-bit Programming Environments Manual for PowerPC, or
>> in the EREF (if you're using a BookE part - e500/e200) for more
>> details.  If you're just learning PowerPC assembler, you should
>> really give this book a good thourough read.
>>
>>>
>>> meerkat wrote:
>>>>
>>>> Good day all,
>>>>
>>>> For the first time I begin working on PPC, and on low level, and
>>>> right
>>>> start from boot sequence, one issue puzzled me.
>>>>
>>>> After bootstrap code (zImage) uncompressed the kernel vmLinux to
>>>> physical
>>>> memory (say from addr 0),
>>>> it jumps to the kernel entry point, _start, using physically  
>>>> address.
>>
>> If you read the book specified above, you will see that branch
>> instructions always specify an effective address, not a physical
>> address.  You can disable translation or map the address so EA=PA,
>> but that's a different issue.
>>
>> -Becky
>> _______________________________________________
>> Linuxppc-embedded mailing list
>> Linuxppc-embedded@ozlabs.org
>> https://ozlabs.org/mailman/listinfo/linuxppc-embedded
>>
>>
>
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>
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