Genode 16.05 Fault handler

Sat Sep 3 12:03:40 CEST 2016

Hello Sebastian,

I think, I have expressed myself incorrectly. I don't want to handle the 
page faults with the same thread which also causes them. I want to find 
out the usage of the new Signal_handler API: I want to create a simple 
component which causes a page fault and which also registers a page 
fault handler for its address space, which wil resolve the page faults 
in a separate Entrypoint (= separate Thread).

Practically, I have created a test scenario [1] with the old 
Signal_receiver and Signal_context syntax, where a component registers a 
page fault handler for its address space and then causes a page fault.

[1] 
https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/7c1c6183a5132aa67d2604221d4eb6ee39106c64/src/test/fault_handler_old/main.cc

Now, I want to change this scenario to use the new Signal_handler 
syntax, where a component registers a Signal_handler and then causes a 
page fault, which is handled by the Signal_handler.

My understanding of a Signal_handler is, that the Entrypoint, which is 
passed at the creation of a Signal_handler, catches a page fault signal 
from core and calls the function passed to the Signal_handler. Because 
an Entrypoint is basically a thread, it will do the same as 
Local_fault_handler class in [1] (Is it correct?).

Based on that thoughts I created this component [2].

[2] 
https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/81f6a705b5b57bc992c47566747a8273fb056c33/src/test/fault_handler/main.cc

I handle the "hen and egg" problem with a separate Entrypoint which is 
actually another thread (other then the thread which causes the page 
fault). But my solution does not work. Where do I have a 
misunderstanding of the Signal_handler/Entrypoint concept?

Basically, I want to know how to handle page faults using a 
Signal_handler object. In my project I will use a parent component, 
which will create several Region_maps and provide one Signal_handler for 
all those Region_maps. One Region_map is given to the child as a 
dataspace, when it allocates ram from its ram_session. The Region_map is 
initially empty. When the child accesses an address in that Region_map, 
the caused page fault will be caught by the Signal_handler and a 
dataspace, backed with physical memory, attached.

Thank you for your help in advance ^^

Kind regards,
Denis

On 31.08.2016 18:23, Sebastian Sumpf wrote:
> Hi Denis,
>
> On 08/31/2016 02:24 PM, Denis Huber wrote:
>> Hello Sebastian,
>>
>> thank you for the working solution :)
>>
>> Now the page faults of threads other than the main-thread can be
>> handled. What about the main-thread? How can I change the example to be
>> able to handle page faults from the main-thread?
>>
>> Do I have to create an extra thread which is dedicated to handle page
>> faults of the other threads like the old Signal_receiver/Signal_context
>> approach [1]? I thought the new design of the Entrypoint provides this
>> feature (Entrypoint handles page faults in its thread). Or is my
>> understanding wrong?
>
> as I said yesterday, a thread (what an EP essentially is) cannot handle
> its own page fault. It is a hen and egg problem. Microkernels just save
> the thread state and call a pager thread in order to resolve the page
> fault. Note, that this thread does not have to be within the same
> program (or protection domain). What I don't understand, why would you
> want a self paging program? The memory has to be provided and paged from
> the outside anyway (in Genode that would be the parent or core)?
>
> Cheers,
>
> Sebastian
>
>>
>> [1]
>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/7c1c6183a5132aa67d2604221d4eb6ee39106c64/src/test/fault_handler_old/main.cc
>>
>>
>> Kind regards,
>> Denis
>>
>> On 30.08.2016 18:36, Sebastian Sumpf wrote:
>>> Hi Denis,
>>>
>>> On 08/30/2016 04:21 PM, Denis Huber wrote:
>>>> Hello Sebastian,
>>>>
>>>> I created a new thread which causes the page fault, and also assigned an
>>>> extra Entrypoint for the Signal_handler [1], but still no success.
>>>> Perhaps I am using the Signal_handler in a wrong way.
>>>>
>>>> [1]
>>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/c58b4144bff720f837112c590ebd2148e9c3f199/src/test/fault_handler/main.cc
>>>>
>>>
>>> You are mixing some things up here, I pushed a working version of your
>>> code (tested on Fisco.OC in Qemu) to GitHub:
>>>
>>> https://github.com/ssumpf/genode/commit/0075a0026faa8079f03dfb4d43ec281ef51f1083
>>>
>>> I hope this clarifies things a little. The relevant output should look
>>> like this:
>>>
>>>> [init -> pf-signal_handler] --- pf-signal_handler started ---
>>>> no RM attachment (WRITE pf_addr=1000 pf_ip=10001d9 from 2d2000)
>>>> Warning: could not resolve pf=0x1000 ip=0x10001d9
>>>> [init -> pf-signal_handler]   Region map state is WRITE_FAULT pf_addr=0x1000
>>>> [init -> pf-signal_handler]   Allocating dataspace and attaching it to the region map
>>>> [init -> pf-signal_handler] --- pf-signal_handler ended ---
>>>
>>> Sebastian
>>>
>>> P.S. As I see it, you needed the 'join' because the thread was created
>>> on the stack again ;) One has to use the heap or bss/data.
>>>
>>>
>>>> Kinds regards,
>>>> Denis
>>>>
>>>> On 30.08.2016 15:45, Sebastian Sumpf wrote:
>>>>> Hello again,
>>>>>
>>>>> I missed that part
>>>>>
>>>>>> *((unsigned int*)0x1000) = 1;
>>>>>
>>>>> in my last reply. You cannot cause a page fault in your entry point and
>>>>> also handle it in the entry point thread, because the entry point is not
>>>>> able to receive signals when it is in page fault. It is a hen and egg
>>>>> problem.
>>>>>
>>>>> If you cause the fault from another Genode thread, it should work as
>>>>> expected.
>>>>>
>>>>> Sebastian
>>>>>
>>>>>
>>>>> On 08/30/2016 03:07 PM, Denis Huber wrote:
>>>>>> Hello Sebastian,
>>>>>>
>>>>>> I moved sigh to the member attributes [1], but without success. I also
>>>>>> tried to use a heap to store the Signal_handler, also without success.
>>>>>>
>>>>>> [1]
>>>>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/93eaec436ce9b6d3b3a6663558bff9af925ae70f/src/test/fault_handler/main.cc#L19
>>>>>>
>>>>>> I am using run script [2], if it helps.
>>>>>>
>>>>>> [2]
>>>>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/93eaec436ce9b6d3b3a6663558bff9af925ae70f/run/fault_handler.run
>>>>>>
>>>>>>
>>>>>> Kind regards,
>>>>>> Denis
>>>>>>
>>>>>> On 30.08.2016 14:36, Sebastian Sumpf wrote:
>>>>>>> Hi Denis,
>>>>>>>
>>>>>>>> Signal_handler<Main> sigh = {env.ep(), *this, &Main::_handle_fault};
>>>>>>>
>>>>>>> The line above, creates the Signal_handler on the stack and destructs
>>>>>>> this handler as soon as the 'Main' constructor is finished.
>>>>>>>
>>>>>>> Therefore, 'sigh' should be a member of the 'Main' class.
>>>>>>>
>>>>>>> Regards,
>>>>>>>
>>>>>>> Sebastian
>>>>>>>
>>>>>>> On 08/30/2016 02:10 PM, Denis Huber wrote:
>>>>>>>> Dear Genode community,
>>>>>>>>
>>>>>>>> in Genode 16.05 the old usage of Signal_receivers and Signal_contexts is
>>>>>>>> considered deprecated. Instead Signal_handler and Signal_transmitter
>>>>>>>> shall be used.
>>>>>>>>
>>>>>>>> I am trying to use a Signal_handler as a fault handler for env's address
>>>>>>>> space. I wrote a short test to illustrate the problem [1].
>>>>>>>>
>>>>>>>> [1]
>>>>>>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/c9a5a1f999a2ce1c6ffd5c316eee127c93d87308/src/test/fault_handler/main.cc
>>>>>>>>
>>>>>>>> The output of [1] is as follows:
>>>>>>>> "
>>>>>>>> [init -> pf-signal_handler] --- pf-signal_handler started ---
>>>>>>>> no RM attachment (WRITE pf_addr=1000 pf_ip=100061c from 2ae000)
>>>>>>>> Genode::Pager_entrypoint::entry()::<lambda(Genode::Pager_object*)>:
>>>>>>>> Could not resolve pf=1000 ip=100061c
>>>>>>>> "
>>>>>>>>
>>>>>>>> The signal handler was registered correctly, because the line
>>>>>>>> "
>>>>>>>> Genode::Core_pd_session_component::submit(Genode::Capability<Genode::Signal_context>,
>>>>>>>> unsigned int)::<lambda(Genode::Signal_context_component*)>: invalid
>>>>>>>> signal-context capability
>>>>>>>> "
>>>>>>>> is missing. However, the method _handle_fault() is never entered. What
>>>>>>>> am I doing wrong?
>>>>>>>>
>>>>>>>>
>>>>>>>> Kind regards,
>>>>>>>> Denis
>>>>>>>>
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