Nope, “infinity” (or any finite approximation ;-) is out in this case. Earlier L4 versions had derivation trees up to 16 deep, and they were a big pain. There’s reason to stick with one, but then should avoid it making effectively zero for some cases (like yours).
Gernot > On 16 Feb 2017, at 10:53, Andrew Gacek <[email protected]> wrote: > > I have no idea how seL4 tracks derivations, but how reasonable is an > answer like 'infinity'? Is anything in seL4 tracked to infinity? How > far are untypeds tracked? > > -Andrew > > On Wed, Feb 15, 2017 at 5:49 PM, <[email protected]> wrote: >> Andrew’s use case makes sense to me at first glance. >> >> I think IRQ caps are special in a way here, as there is a difference to >> other derived caps: A cap for a single IRQ is logically a top-level cap, >> similar to a frame cap. This present model basically means that you can’t >> delegate them, unlike other objects. Seems like a weakness (if not >> conceptual inconsistency) in our present model. >> >> As Gerwin indicates, just moving to two levels is not necessarily a good >> solution. I tend to think that the only valid magic numbers are zero, one, >> and infinity ;-) >> >> Gernot >> >>> On 16 Feb 2017, at 10:31, [email protected] wrote: >>> >>> Currently, this is mostly implementation driven - there is one bit reserved >>> for the derivation level in the data structure that tracks it. It’s >>> possible that IRQControl caps specifically have some space left that could >>> be used for more levels, but it would make them a special case. >>> >>> If we reserved 2 bits for the level, you’d hit the same problem somewhat >>> later, though, and the argument at the time was that (very small) >>> finiteness of derivation levels of these control caps has to be solved at >>> user level anyway and it’s better to make you think of it immediately >>> rather than when you’ve designed yourself into a corner. >>> >>> Maybe you do have a very good use case here, though, and we should rethink >>> that argument (as we did for endpoint caps - their level of specialness is >>> pretty messy, but we considered it worth the pain). I should probably leave >>> that part to Kevin. >>> >>> Cheers, >>> Gerwin >>> >>>> On 16.02.2017, at 03:20, Andrew Gacek <[email protected]> wrote: >>>> >>>> Based on the seL4 manual it sounds like IRQControl caps only support >>>> one level of derivation. What is the reason for this restriction? We >>>> encountered a case where we wanted to hand out an IRQControl for a >>>> specific irq and then later revoke access, but we couldn't do it >>>> because the IRQControl for a specific irq is already a derived >>>> capability. >>>> >>>> -Andrew >>>> >>>> _______________________________________________ >>>> Devel mailing list >>>> [email protected] >>>> https://sel4.systems/lists/listinfo/devel >>> >>> _______________________________________________ >>> Devel mailing list >>> [email protected] >>> https://sel4.systems/lists/listinfo/devel >> _______________________________________________ >> Devel mailing list >> [email protected] >> https://sel4.systems/lists/listinfo/devel _______________________________________________ Devel mailing list [email protected] https://sel4.systems/lists/listinfo/devel
