Hi Oak.
Yes, I think Gerwin has missed that frame caps are a bit of a special case.
Per-object finalisation actions happen when the last cap to an object is being
removed. You can make as many copies of an endpoint cap as you like, and the
endpoint will only be cleared when the last one is deleted. Typically the
creating manager/supervisor keeps the original for itself and decides when to
perform this action.
Some caps have per-cap state as well, e.g. the mapping info stored in a frame
cap. As an aside, frame caps are the only caps with per-cap state that I know
of, but other architectures might have more. This per-cap state is finalised
with the cap. So deleting a mapped copy (or deleting all mapped copies with the
Revoke operation) will remove the mapping. The kernel has to do this, in fact,
because information about how to remove the mapping only exists in the mapped
cap, which is the whole point of forcing you to duplicate the caps per-mapping.
Cheers,
Thomas.
On 23/06/17 05:24, Norrathep Rattanavipanon wrote:
Hi Gerwin,
Thank you for your response. But I'm a little bit confused here. Maybe I
misunderstood something or I didnt phrase the question correctly.
From my understanding (please correct me if I'm wrong), each frame cap can only
be mapped once.
This means, you cannot have multiple mappings created by the same frame cap.
Then what did you mean by:
"the kernel will unmap the page if the cap that you are deleting is the last
cap to it. If there is still a copy of the frame cap around, it will not
automatically unmap."
?
Oak
On Thu, Jun 22, 2017 at 12:43 AM,
<[email protected]<mailto:[email protected]>> wrote:
Hi Oak,
the kernel will unmap the page if the cap that you are deleting is the last cap
to it. If there is still a copy of the frame cap around, it will not
automatically unmap.
This is a general principle for cap deletion: deleting the last cap to an
object triggers object “finalisation”, i.e. cleanup and resetting it to an
inert state so that it can later be removed from memory without impacting the
rest of the system. If there are still other caps to the same object in the
system, only the cap is removed.
Cheers,
Gerwin
> On 22.06.2017, at 13:50, Norrathep Rattanavipanon
> <[email protected]<mailto:[email protected]>> wrote:
>
> Hello,
>
> I was wondering when we call seL4_cnode_delete on a (mapped) frame cap,
> does the kernel also handle unmapping the frame (in addition to withdrawing
> authority) as well?
> Or the user-space has to ensure that the frame is unmapped first before
> calling delete?
>
> I tried my code without unmapping that frame when deleting the cap and it
> seems to work fine.
> So I guess the kernel handles that?
>
> Oak
>
> --
> Norrathep (Oak) Rattanavipanon
> M.S. in Computer Science
> University of California - Irvine
> _______________________________________________
> Devel mailing list
> [email protected]<mailto:[email protected]>
> https://sel4.systems/lists/listinfo/devel
--
Norrathep (Oak) Rattanavipanon
M.S. in Computer Science
University of California - Irvine
_______________________________________________
Devel mailing list
[email protected]<mailto:[email protected]>
https://sel4.systems/lists/listinfo/devel
_______________________________________________
Devel mailing list
[email protected]
https://sel4.systems/lists/listinfo/devel
