Checkpoint/restore of capabilities

Denis Huber huber.denis at ...435...
Tue Oct 18 18:25:04 CEST 2016

Hello Norman,

you are right, it is quite complicated, but I think I understand the 
capability concept in Genode with Fiasco.OC. Let me recap it:

I created a simple figure [1] to illustrate my thoughts. A component has 
a capability map and a kernel-intern capability space. Each managed RPC 
object has a capability which points to a capability map slot that 
stores a system-global identifier called badge. The capability space 
slot can be computed through the capability map slot. The corresponding 
capability map slot points to the object identity which is an IPC gate.


In order to restore a component on another ECU, the checkpointed 
variables representing capabilities (entries in memory, e.g. stack) have 
to be made valid. Therefore, I have to restore the IPC gate, the 
capability space slot pointing to this IPC gate, and allocate a new 
badge, because it is valid only in one system and the component is 
migrated to another system. Also, I have to restore the capability map 
slot to point to the new badge and restore the RPC object.

In the following I assume that the RPC objects of the target component 
are created by the Checkpoint/Restore component (i.e. it intercepts the 
session requests and provides own sessions at child creation). The other 
case regarding local RPC objects of the target component will be 
discussed later, if I hopefully have the time:

By virtualizing the session RPC objects and the normal RPC objects, I 
can checkpoint the state of them. Thus, I can recreate an RPC object. 
When I do that the RPC object has a new capability (local to the 
Checkpoint/Restore component) and a valid badge. Implicitly a valid IPC 
gate is also recreated. Thus, the target component has to know this 
capability inside its protection domain. Therefore, the capability 
space/map slot has to point to the IPC gate or to the new badge, 
* The capability space slot is recreated by issuing l4_task_map to map a 
capability from core to the target child. This is done by extending 
Foc_native_pd interface (see in an earlier mail from Norman).
* The capability map slot is recreated by 
Capability_map::insert(new_badge, old_kcap). Thus, I have to checkpoint 
the kcap by Capability_map::find(new_badge)->kcap().

Now I am missing the pointer to target component's internal capability 
map. I already have all dataspace capabilities which are attached to the 
target's address space. With the pointer I can cast it to a 
Capability_map* and use its methods to manipulate the Avl-tree. Please 
correct me if I am wrong.

Norman, you proposed a rough idea of how to obtain a dataspace 
capability of the capability map through the PD_session in one of your 
previous mails:

On 07.10.2016 09:48, Norman Feske wrote:
 >2. We may let the child pro-actively propagate information about its
 >   capability space to the outside so that the monitoring component can
 >   conveniently intercept this information. E.g. as a rough idea, we
 >   could add a 'Pd_session::cap_space_dataspace' RPC function where a
 >   component can request a dataspace capability for a memory buffer
 >   where it reports the layout information of its capability space.
 >   This could happen internally in the base library. So it would be
 >   transparent for the application code.

Can you or of course anyone else elaborate on how it "could happen 
internally in the base library"? Does core know the locations of 
capability maps of other components?

Kind regards,

PS: If my thoughts contain a mistake, please feel free to correct me. It 
would help me a lot :)

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