Thread switching in Genode/seL4 and registers and utcb access problem

[Adam Wiethuechter]

Alex,

I would encourage passing what you found along (even the documentation 
improvement suggestion) to Data61. They are very open to any form of 
feedback with seL4.

On 9/26/2019 6:53 AM, Alexander Tormasov via users wrote:
>>
>> Problem 2.
>> When I start trying to make a fast solution and try to read registers 
>> and some info from low level physical seL4 thread 
>> using seL4_TCB_ReadRegisters I found  that:
>> In returned successfully data rip register is always 0 for current 
>> thread, tcb selector myself.native_thread().tcb_sel also 0, and rsp 
>>  = 0x13!
>> Probably I can’t read myself?
>>
> Seems that I found answer in the seL4 code. They do not allow 
> ReadReginster from myself:
>
> In file object/tcb.c
> exception_t
> decodeReadRegisters(cap_t cap, word_t length, bool_t call,
>                     word_t *buffer)
> {
> …
>    thread = TCB_PTR(cap_thread_cap_get_capTCBPtr(cap));
>     if (thread == NODE_STATE(ksCurThread)) {
>         userError("TCB ReadRegisters: Attempted to read our own registers.");
>         current_syscall_error.type = seL4_IllegalOperation;
>         return EXCEPTION_SYSCALL_ERROR;
>     }
>
> Seems that this is my fault (while this is not anyhow specified in the 
> docs, only some innuendo about stopped thread read)
>
> So, the  function used from library seL4_TCB_ReadRegisters 
> incorrectly return 0 - everything is ok while it is not.
>
> Also seems that seL4_TCB_WriteRegisters do not work in such conditions 
> as well - it return without errors and not changing rip to new 
> function specified in appropriate field …
>
>
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-- 
73's,
Adam Wiethuechter, Junior Software Engineer
Critical Technologies Inc. (CTI)
Desk: (315)-793-0248 x157
Cell: (315)-552-4298
<adam.wiethuechter at critical.com>

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