Re: understanding head.S

[David VomLehn <dvomlehn@cisco.com>]

Wang, Baojun wrote:
> 在 2008-06-05四的 15:32 +0530，Chetan Nanda写道：
>   
>> Hi All,
>> I was reading the boot code for MIPS.
>> in head.S file before jumping to 'start_kernel' it calculate the stack
>> pointer address as follow:
>>
>> "
>> 	PTR_LA		$28, init_thread_union
>> 	PTR_ADDIU	sp, $28, _THREAD_SIZE - 32
>>     
> make sp point to the stack top so that we can allocate stack space
> later, -32 is a reserved area for local variable, saved GPR, arguments
> (function stack frame).. due to the abi
>   
The MIPS ABI, whose official name is "System V Application Binary 
Interface: MIPS(r) Processor Supplement, 3rd Edition", may be found at 
http://math-atlas.sourceforge.net/devel/assembly/mipsabi32.pdf.. The 
code generated by gcc does not conform to the MIPS ABI in all cases, but 
it is the closest thing we have to a standard.

The init_thread_union item is a pre-allocated stack area. The PTR_LA 
loads the address of the beginning of that item, but since the stack 
grows down, toward smaller addresses, we need to set the stack pointer 
to the highest address in the stack area. I'm pretty sure the MIPS ABI 
doesn't mention anything about the 32-byte area as it doesn't address 
the case of  how kernel stacks should be initialized. It's not clear to 
me what it's there for, but it might be helpful for detecting stack 
underflows or for giving a little extra space before a buffer overflow 
corrupts other kernel data structures.
>> 	set_saved_sp	sp, t0, t1
>>     
> set_saved_sp is defined (as a assembler macro) in
> include/asm-mips/stackframe.h (if you're using vim, I think it will be
> easy to use vim with cscope..)
>   
We must load the stack pointer with pointer to a valid kernel stack 
before calling any C code to handle interrupts or exceptions. The 
assembly language macro set_saved_sp is how we store the kernel stack 
pointer to use in such a case. The SAVE_SOME macro is generally used to 
restore the stack pointer.
>> 	PTR_SUBU	sp, 4 * SZREG		# init stack pointer
>>     
> SZREG is defined to the general purpose register size, it is 32bit in
> MIPS32 and 64bit in MIPS64, so the last line allocates 16/32 bytes to
> build a stack frame.. You could refer to the MIPS ABI if you need
> further information (the kernel don't follow the standard ABI all the
> time though..)
>   
The MIPS ABI states that the caller of a function must reserve space for 
four arguments, which are passed in the registers a0-a3, even if the 
function being called is not being passed four arguments. Since SZREG is 
the size of one register, 4 * SZREG reserves enough space for these four 
arguments. See the section "C Stack Frame" on page 3-46 and Figures 3-37 
and Figure 3-38 in the MIPS ABI for details about what C stack frames 
look like.
>> Can anyone please explains me this 4 lines of code?
>> Why ' _THREAD_SIZE - 32' is added in 'sp' ?
>> What 'set_saved_sp' will do ?
>> and then why we subtract '4 * SZREG' from 'sp' ?
>>
>> Please hep me to understand this code better.
>>
>> Thanks,
>> Chetan Nanda
>>