Restoring child with checkpointed state

David Werner wernerd at ...389...
Wed Jun 7 15:13:45 CEST 2017

Hi everyone,

after Denis Huber left the project, I am in charge of making our 
checkpoint/restore component work.
Therefore i would like to ask some more questions on the IRQ kernel object.

1. When is the IRQ object created? Does every component have an own IRQ 

I tried to figure out when the IRQ object is mapped into the object 
space of a component on its startup. Therefore I took a look at the code 
in [repos/base-foc/src/core/]. The IRQ object 
appears in the object space after the "_sem = 
<Rpc_request_semaphore>();" statement in the constructor.

As far as I could  follow the implementation the "request_semaphore" RPC 
call is answered by the "Signal_source_rpc_object" in 
[base-foc/src/include/signal_source/rpc_object.h] which 
returns/delegates the native capability "_blocking_semaphore" which is 
an attribute of the "Signal_source_rpc_object". It seems to me that the 
IRQ object already exists at this point and is only delegated to the 

But when is the IRQ object created and by whom? Is it created when a new 
PD session is created?

2. Does the IRQ object carry any information? Do I need to checkpoint 
this information in order to be able to recreate the object properly 
during a restore process? Is the IRQ object created automatically (and i 
only have to make sure that the object is getting mapped into the object 
space of the target) or do i have to create it manually?

In our current implementation of the restore process we restore a 
component by recreating its sessions to core services (+timer) with the 
help of information we gathered using a custom runtime environment. 
After the sessions are restored we place them in the object space at the 
correct position. Will I also have to somehow store information about 
the IRQ object? Or is it just some object that needs to exist?

Kind Regards,

Am 29.03.2017 um 14:05 schrieb Stefan Kalkowski:
> Hello Dennis,
> On 03/27/2017 04:14 PM, Denis Huber wrote:
>> Dear Genode community,
>> Preliminary: We implemented a Checkpoint/Restore mechanism on basis of
>> Genode/Fiasco.OC (Thanks to the great help of you all). We store the
>> state of the target component by monitoring its RPC function calls which
>> go through the parent component (= our Checkpoint/Restore component).
>> The capability space is indirectly checkpointed through the capability map.
>> The restoring of the state of the target is done by restoring the RPC
>> objects used by the target component (e.g. PD session, dataspaces,
>> region maps, etc.). The capabilities of the restored objects have to be
>> also restored in the capability space (kernel) and in the capability map
>> (userspace).
>> For restoring the target component Norman suggested the usage of the
>> Genode::Child constructor with an invalid ROM dataspace capability which
>> does not trigger the bootstrap mechanism. Thus, we have the full control
>> of inserting the capabilities of the restored RPC objects into the
>> capability space/map.
>> Our problem is the following: We restore the RPC objects and insert them
>> into the capability map and then in the capability space. From the
>> kernel point of view these capabilities are all "IPC Gates".
>> Unfortunately, there was also an IRQ kernel object created by the
>> bootstrap mechanism. The following table shows the kernel debugger
>> output of the capability space of the freshly bootstraped target component:
>> 000204 :0016e* Gate   0015f* Gate   00158* Gate   00152* Gate
>> 000208 :00154* Gate   0017e* Gate   0017f* Gate   00179* Gate
>> 00020c :00180* Gate   00188* Gate          --            --
>> 000210 :       --            --     0018a* Gate   0018c* Gate
>> 000214 :0018e* Gate   00196* Gate   00145* Gate   00144* IRQ
>> 000218 :00198* Gate          --            --            --
>> 00021c :       --     0019c* Gate          --            --
>> At address 000217 you can see the IRQ kernel object. What does this
>> object do, how can we store/monitor it, and how can it be restored?
>> Where can we find the source code which creates this object in Genode's
>> bootstrap code?
> The IRQ kernel object you refer to is used by the "signal_handler"
> thread to block for signals of core's corresponding service. It is a
> base-foc specific internal core RPC object[1] that is used by the signal
> handler[2] and the related capability gets returned by the call to
> 'alloc_signal_source()' provided by the PD session[3].
> I have to admit, I did not follow your current implementation approach
> in depth. Thereby, I do not know how to exactly handle this specific
> signal hander thread and its semaphore-like IRQ object, but maybe the
> references already help you further.
> Regards
> Stefan
> [1] repos/base-foc/src/core/
> [2] repos/base-foc/src/lib/base/
> [3] repos/base/src/core/include/pd_session_component.h
>> Best regards,
>> Denis
>> On 11.12.2016 13:01, Denis Huber wrote:
>>> Hello Norman,
>>>> What you observe here is the ELF loading of the child's binary. As part
>>>> of the 'Child' object, the so-called '_process' member is constructed.
>>>> You can find the corresponding code at
>>>> 'base/src/lib/base/'. The code parses the ELF executable
>>>> and loads the program segments, specifically the read-only text segment
>>>> and the read-writable data/bss segment. For the latter, a RAM dataspace
>>>> is allocated and filled with the content of the ELF binary's data. In
>>>> your case, when resuming, this procedure is wrong. After all, you want
>>>> to supply the checkpointed data to the new child, not the initial data
>>>> provided by the ELF binary.
>>>> Fortunately, I encountered the same problem when implementing fork for
>>>> noux. I solved it by letting the 'Child_process' constructor accept an
>>>> invalid dataspace capability as ELF argument. This has two effects:
>>>> First, the ELF loading is skipped (obviously - there is no ELF to load).
>>>> And second the creation of the initial thread is skipped as well.
>>>> In short, by supplying an invalid dataspace capability as binary for the
>>>> new child, you avoid all those unwanted operations. The new child will
>>>> not start at 'Component::construct'. You will have to manually create
>>>> and start the threads of the new child via the PD and CPU session
>>>> interfaces.
>>> Thank you for the hint. I will try out your approach
>>>> The approach looks good. I presume that you encounter base-foc-specific
>>>> peculiarities of the thread-creation procedure. I would try to follow
>>>> the code in 'base-foc/src/core/' to see what the
>>>> interaction of core with the kernel looks like. The order of operations
>>>> might be important.
>>>> One remaining problem may be that - even though you may by able the
>>>> restore most part of the thread state - the kernel-internal state cannot
>>>> be captured. E.g., think of a thread that was blocking in the kernel via
>>>> 'l4_ipc_reply_and_wait' when checkpointed. When resumed, the new thread
>>>> can naturally not be in this blocking state because the kernel's state
>>>> is not part of the checkpointed state. The new thread would possibly
>>>> start its execution at the instruction pointer of the syscall and issue
>>>> system call again, but I am not sure what really happens in practice.
>>> Is there a way to avoid this situation? Can I postpone the checkpoint by
>>> letting the entrypoint thread finish the intercepted RPC function call,
>>> then increment the ip of child's thread to the next command?
>>>> I think that you don't need the LOG-session quirk if you follow my
>>>> suggestion to skip the ELF loading for the restored component
>>>> altogether. Could you give it a try?
>>> You are right, the LOG-session quirk seems a bit clumsy. I like your
>>> idea of skipping the ELF loading and automated creation of CPU threads
>>> more, because it gives me the control to create and start the threads
>>> from the stored ip and sp.
>>> Best regards,
>>> Denis
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