[okl4-developer] About the virtual memory management of OKL4

[Ben Leslie]

On Sat Nov 17, 2007 at 18:19:52 +0800, kashin Lin wrote:
>Hi,
>
>i have read okl4-progmanual & iguana-usermanual, but confused their memory
>management.
>
>in l4 microkernel, threads in the same address space belong to the same
>process(task). so it means
>there will be various address
>space. but with iguana, all threads belong to the same process(task) are
>in the same
>address space with different PD. all thread created by iguana are also
>l4 threads ,
>why those difference exist?

How the microkernel and the root task relate is a confusing point of
L4.

The root task, in this case Iguana, is free to present its own
abstraction to user programs. It uses the underlying microkernel to
implement those abstractions.

In this case, each Iguana protection domain is implemented by an
underlying L4 address space. 

It happens that Iguana ensures that all these L4 address spaces have
the same virtual -> physical mapping. (Although they do have different
permissions on the virtual memory.)


>in l4 microkernel, threads can communicatin between different or same  AS by
>l4_IPC. but
>according to iguana manual, why it says "Explicit L4 mapping IPC is not
>permitted between Iguana
>protection domains". threads created with iguana are in the same AS, aren't
>they?

This is a case of the usermanuals being slightly out of date. Older
versions of the L4 API allowed memory to be mapped between L4 address
space by sending a "mapping IPC". To ensure the single-address-space
model is preserved, Iguana did not allow this operation to occur.

The latest versions of L4 do not have this feature any more, so the comment
in the Iguana manual is redundant.

>if i created a thread, what's are the address space it can access (PHY
>addr)?
>either creating a thread "directly on L4" or "with iguana", will the
>PHY address
>it can access be different?

In L4 the only way to create a thread is have the root-task create it.
So, if you are using Iguana, only Iguana is allowed to directly create
a thread. Any other programs that wish to create threads must ask
Iguana to do this on their behalf.

>(in okl4-progmanual, it says "This address space is initialised at boot time
>by the microkernel based
>on the initial mapping descriptors in the KIP memory descriptors as
>described in Chapter B-5."
>but it's missing)

This refers to the address initial root task address space. Any other
address spaces have their mappings initialised by the root task itself.

>maybe these quetions due
>to my less comprehend of the relationship between l4 & iguana.
>so it will be better if you can show me that.
>
>i really need to comprehend those. any help will be welcome!
>thanks in advance~

Hopefully this has helped explain things, but maybe not. If not please
feel free to write again, and I will try to clarify any points.

Cheers,

Benno