On Sun, Feb 3, 2013 at 10:10 PM, Stefan Kalkowski <stefan.kalkowski@...1...> wrote:

Hi Jaeyong,

On 2013-02-01 03:47, jaeyong yoo wrote:
> Thanks a lot. Stefan.
> I think it is a very nice article for understanding Fiasco.OC/Genode
> IPC and capability!
> Also, reading the code, "base-foc/include/base/ipc.h," is much easier
> and clearer after your kind explanation.
>
> I would like to ask the initial question once again (I'm strongly
> feeling that I have to buy you beer :) If we could meet in person,
> I'm
> gonna!).

I'll take you at your word ;-)

> I think it is the part that you mentioned "reality is even more
> complex."
>
> In the function, Capability_map::insert_map(int id, addr_t kcap),
> there is a part that allocating a new capability index:
>
> 141         /* if we own the capability already check whether
> it's the same */
> 142         if (i) {
> 143                 l4_msgtag_t tag =
> l4_task_cap_equal(L4_BASE_TASK_CAP, i->kcap(), kcap)
> ;
> 144                 if (!l4_msgtag_label(tag)) {
> 145                         /*
> 146                          * they aren't
> equal, possibly an already revoked cap,
> 147                          * otherwise
> it's a fake capability and we return an invalid o
> ne
> 148                          */
> 149                         tag =
> l4_task_cap_valid(L4_BASE_TASK_CAP, i->kcap());
> 150                         if
> (l4_msgtag_label(tag))
> 151
> return 0;
> 152                         else
> 153                                 /*
> it's invalid so remove it from the tree */
> 154
> _tree.remove(i);
> 155                 } else
> 156                         /* they are equal
> so just return the one in the map */
> 157                         return i;
> 158         }
> 159
> 160         /* the capability doesn't exists in the map so
> allocate a new one */
> 161         i = cap_idx_alloc()->alloc_range(1);
>
> It allocates a new capability index, if the already existing
> capability is invalid (see line # 153).
> What I'm wondering is the following:
> Since the 'kcap' (parameter to insert_map func.) which is valid
> capability index received from the IPC (and originally allocated in
> Msgbuf_base creator), would it be possible to use the kcap rather
> than
> allocating the new one?
>

Actually it is possible, but then we must instead allocate a new
capability index within the "Msg_buf" object, from where the "kcap"
argument originally comes from.
As stated in a previous mail: the capability indices in the message
buffer are used to receive new capabilities. If we just use these
indices without allocating new ones, newly received capabilities would
override the old ones.

Does that answers your question?

Best regards
Stefan

> Best regards,
> Jaeyong
>
> On Fri, Feb 1, 2013 at 4:20 AM, Stefan Kalkowski

> <stefan.kalkowski@...1... [21]> wrote:
>
>> Hi Jaeyong,
>>
>> On 01/31/2013 03:59 AM, jaeyong yoo wrote:
>> >> 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;
>> > }
>>
>> Well, I suppose that's the point where things get wrong ;-).
>> I fear you're still mixing up Genode and Fiasco.OC abstraction
>> layers.
>> Whereby, I've to admit the topic is quite complicated.
>>
>> On the kernel API level a capability is just an address value
>> (denoted
>> as "dst" above). In Genode a capability consists of the kernel
>> capability and an additional value (denoted as "local_name"). When
>> transfering a Genode capability via the kernel's IPC mechanism, you
>> transfer the "local_name" as a normal value. Whereby the kernel
>> capability is handled separately.
>> When waiting for, or sending a message via the kernel's IPC
>> primitives,
>> you've to describe what kind of items you'll send or receive via
>> the
>> UTCB (user-level thread control block). Therein you describe how
>> many
>> normal data values, memory mappings, or kernel capability mappings,
>> you're willing to receive or to send.
>>
>> Imagine a thread wants to receive a single capability. Then it has
>> to
>> tell the kernel where in its task's capability space the received
>> item
>> should end up. The kernel isn't responsible for allocating/freeing
>> items
>> in the capability space. The same way like most microkernels aren't
>> in
>> authority to organize the memory spaces, but let the userland
>> organize
>> their virtual memory themself. So lets say the thread wants the
>> capability to end up at 0x13000 in its capability space. Then it
>> would
>> prepare its UTCB e.g. like the following:
>>
>> l4_utcb_br()->br[0] = 0x13000 | L4_RCV_ITEM_SINGLE_CAP;
>>
>> before it triggers an ipc_wait syscall. If on the other side the
>> sender
>> put a capability into its UTCB in an appropriated manner, the
>> kernel
>> will now associate the kernel object behind the capability to be
>> send
>> with address 0x13000 in the capability space of the receiving
>> thread.
>> That means the transfered capability ends up at 0x13000. If the
>> thread
>> wants to invoke the received capability it will use this address
>> for it.
>>
>> The "local_name" value, which is only meaningful for the Genode
>> abstractions, has to be transfered like every other normal values,
>> like
>> e.g. opcodes, strings, etc.. These values are stored in a different
>> area
>> of the UTCB. The kernel doesn't care about it.
>>
>> After successfully receiving an IPC on the kernel level, within
>> Genode
>> we transfer the values out of the UTCB of the thread, into the
>> "Msg_buf"
>> object - a Genode abstraction. This message buffer again consists
>> of
>> normal data, hold in a byte array, and kernel capability indices,
>> hold
>> in a corresponding array. Now, if you unmarshal a Genode capability
>> out
>> of such a message buffer object, you will take the "local_name"
>> value
>> out of the normal data array, and the "dst" value out of the
>> capability
>> indices array,and construct a Genode capability out of it.
>>
>> I've to admit, reality is even more complex, but this is the
>> reasonably
>> neat story. If you want to have a look, how Genode capabilities are
>> constructed out of the message buffer in more detail, please have a
>> look
>> at: base-foc/include/base/ipc.h
>>
>> I think this whole topic would be better off in a separate article.
>> Nevertheless, I hope my explanatory notes are more enlightening
>> than
>> confusing.
>>
>> Best regards
>> Stefan
>>
>> >
>> > 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... [1]

>>
>>> <mailto:stefan.kalkowski@...1... [2]>> 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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>> > >
>> > >
>> > > _______________________________________________
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>> > > Genode-main@lists.sourceforge.net [4]
>> > <mailto:Genode-main@lists.sourceforge.net [5]>
>>
>>> > https://lists.sourceforge.net/lists/listinfo/genode-main
>> [6]

>> > >
>> >
>> > --
>> > Stefan Kalkowski
>> > Genode Labs
>> >

>> > http://www.genode-labs.com/ [7] · http://genode.org/ [8]

>> >
>> >
>>
>
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>>
>>>
>> >
>> >
>> >
>> >
>>
>
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>> >
>>
>> --
>> Stefan Kalkowski
>> Genode Labs
>>

>> http://www.genode-labs.com/ [16] · http://genode.org/ [17]

>>
>>
>
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>
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--
Stefan Kalkowski
Genode Labs

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