How to get saved registers of a process in Linux (ARM Architecture)

I'm writing a program that parses all tasks running on linux, starting from init_task for each task i read it's task_struct structure which allows me to get it's PID, State, Oncpu..

However i also need to find the saved registers of that task, especially registers R0 to R10, IP, SP, FP and PC

Also in task_struct I found a pointer to a structure called cpu_context which holds registers R4 to PC

So the problem is that I don't know how to get registers R0 to R3 I tried to manually parse the stack of the task but i haven't found any relevant values

so here are my questions:

-where in the stack(or in an other location in memory) are saved the registers of a task that is not running?

-Can I trust the values of the registers R4 to PC found in the structure cpu_context?

I'm using a board containing开发者_开发问答 an ARM Cortex A9 MPCore processor(2 cores), linked with the host PC with a JTAG Link

Linux Kernel 2.6.35.7+ is running on the board(of course this kernel was compiled for the ARM architecture)

On the Host PC i'm using OPENOCD and GDB for the debug.

Thanks

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It depends on which set of registers you are interested in.

If you are interested in the user mode state, take a look on how ptrace does it. From a quick peek at the source code, task_pt_regs(task) is where you should look. Apparently, they are near the top of the kernel stack for the task (take a look at vector_swi for instance; it has a stmia sp, {r0 - r12} near its beginning, followed by a store of sp and lr).

If you are interested in the kernel mode state, it is saved by __switch_to into task->cpu_context (TI_CPU_SAVE is the offset of cpu_context within the struct thread_info). As another answer already noted, it doesn't save r0-r3 because it doesn't have to; the caller of switch_to assumes they will be clobbered by __switch_to, so their values don't matter.

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cpu_context contains the value of registers when __switch_to is called, caller saved registers are not stored.

If you want the value of registers upon entry to an interrupt or system call you need to look elsewhere.