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.
[1] https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b78f5...
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, respectively. * 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:
- 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, Denis
PS: If my thoughts contain a mistake, please feel free to correct me. It would help me a lot :)