Reading thread registers

Christian Prochaska christian.prochaska at ...1...
Fri Aug 19 16:28:31 CEST 2016

Hi Denis,

On 19.08.2016 10:30, Denis Huber wrote:
> I tried reading the registers through the "Thread_state 
> Cpu_thread::state()" method, but it did not contain any information 
> about (just zeros).
> I'm using Genode 16.05, kernel Fiasco.OC (pbax9 build). By inspecting 
> the kernel API of foc, I found out that the function 
> l4_thread_ex_regs_ret can read the sp and ip register.

On Fiasco.OC, the 'Platform_thread::pause()' function forces the thread into an exception state, which is handled by the pager, who
reads the register state (including r0-r12) from the UTCB in 'Ipc_pager::_parse()'. The 'Platform_thread::state()' function then
returns this previously saved state. If you called 'Cpu_thread::pause()' before 'Cpu_thread::state()' and did not get the correct
register state, perhaps the thread was just executing a syscall. In that case, the exception would only occur after the syscall
returned, which might take a long time, so 'Platform_thread::pause()' returns without waiting for the exception when it detects this
situation. A possible solution for this problem without having to block until the thread returns from the syscall might be to extend
'l4_thread_ex_regs_ret()' to return the missing registers in the UTCB.


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