Re: [microblaze-uclinux] newbie question - KSP in entry.S

[John Williams <jwilliams@xxxxxxxxxxxxxx>]

Hi Gary,

Gary Schumacher wrote:
> We are using uClinux (2.4.32-uc0) on a Microblaze processor here and am 
> hunting down a problem.  In doing so, I am looking at entry.S and have a 
> question how KSP is used.  It is first set in head.S and is read in 
> SAVE_STATE and written in RETURN.  On the surface one would assume that 
> KSP would maintain a value close to its original value (+- SAVE_SIZE) 
> but I see it changing greatly.  Could somebody enlighten me on what I am 
> missing on the usage of KSP in 2.4? 

Each Linux process has two stacks - one userspace, and one kernel space. 
  KSP is a kernel global variable holds the value of the kernel stack 
pointer, for the current process, while it is running in usermode.  It 
is required because unlike some other CPUs, MicroBlaze doesn't have 
seperate system and user stack pointer registers.

In a nutshell you see many different values because each process has its 
own kernel stack.

There's a good description of this stuff in "Understanding the Linux 
Kernel", but you need to read it a few times before it makes sense!

Cheers,

John

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