When waiting for IPC you can specify capability indices, which will contain capabilities passed by the sender. If the capability indices you specified contain still valid capabilities, they will nevertheless get overwritten. In Genode, the message buffer object "Msg_buf" has some capability indices used to hold received capabilities. Before receiving new ones, already received capabilities are duplicated via "l4_task_map".
Thanks for the answer. I would like to check that I understand correctly with the following example. Please stop me if I am going somewhere wrong.
We can consider a thread waiting for an IPC with the following rcv_buf:
rcv_buf { [cap: 0x13000(dst), 0x002f(local_name)] some buf; }
The thread receives an IPC call and the rcv_buf becomes the following:
rcv_buf { [cap: 0x14000, 0x002f] // <--- overwritten to 0x14000. some buf; } And, 0x14000 and 0x13000 is mapped by l4_task_map. And, the thread who waits for the IPC call (who originally created 0x13000) uses the capability index 0x13000 as usual.
Regards, Jaeyong
On Wed, Jan 30, 2013 at 7:34 PM, Stefan Kalkowski < stefan.kalkowski@...1...> wrote:
Hi Jaeyong,
On 01/30/2013 02:20 AM, jaeyong yoo wrote:
Hello! Genode,
From time to time, I saw something like this:
l4_task_map(L4_BASE_TASK_CAP, L4_BASE_TASK_CAP, l4_obj_fpage(A.dst(), 0, L4_FPAGE_RWX), B.dst() | L4_ITEM_MAP);I think it looks like a unnecessary mapping, because rather than mapping, we can just copy the capability like this, A = B.
No, that's not the same. Moreover, it has to be "B = A" in your example.
The "l4_task_map" call above, creates a duplicate of the capability referenced by "A.dst()" to "B.dst()". The value given by "dst()" is an index into the capability name space of the task - in the following shortly titled as capability index. You cannot modify the capability name space in user mode. All changes to that space are done explicitly via the "l4_task_map" syscall, or implicitly by sending a capability mapping via IPC.
The second example is simply an assignment, where A and B will reference the same capability index in the end. Moreover, the reference counter in Genode's capability will be increased. Look at "base-foc/include/base/native_types.h" for the definition of the capability class for Fiasco.OC.
Normally, we simply use the simple assignments when passing around capabilities by value. But in some rare situations it is necessary to really duplicate a capability index. When waiting for IPC you can specify capability indices, which will contain capabilities passed by the sender. If the capability indices you specified contain still valid capabilities, they will nevertheless get overwritten. In Genode, the message buffer object "Msg_buf" has some capability indices used to hold received capabilities. Before receiving new ones, already received capabilities are duplicated via "l4_task_map".
Regards Stefan
Since I'm feeling like I miss something here, could you tell me the difference between two of them?
Best regards, Jaeyong
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