Looks like unnecessary mapping of capability

[jaeyong yoo]

> 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 at ...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
> >
> >
> >
> ------------------------------------------------------------------------------
> > Everyone hates slow websites. So do we.
> > Make your web apps faster with AppDynamics
> > Download AppDynamics Lite for free today:
> > http://p.sf.net/sfu/appdyn_d2d_jan
> >
> >
> >
> > _______________________________________________
> > Genode-main mailing list
> > Genode-main at lists.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/genode-main
> >
>
> --
> Stefan Kalkowski
> Genode Labs
>
> http://www.genode-labs.com/ · http://genode.org/
>
>
> ------------------------------------------------------------------------------
> Everyone hates slow websites. So do we.
> Make your web apps faster with AppDynamics
> Download AppDynamics Lite for free today:
> http://p.sf.net/sfu/appdyn_d2d_jan
> _______________________________________________
> Genode-main mailing list
> Genode-main at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/genode-main
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.genode.org/pipermail/users/attachments/20130131/6122150f/attachment.html>