Re: Hurd Advocacy?
On Sun, Aug 17, 2003 at 05:27:32PM +0100, Mark Wilkinson wrote:
> If the decision has been made to go to L4, IMHO work should have halted
> on Mach.
People have halted on Mach long before L4 even entered the picture.
> Forgive my ignorance, but why is L4 3-5 years away?
L4 was released a month ago or so. But it is not a drop in replacement for
Mach.
> If the
> decision has been made to move to a kernel structure that doesn't exist
> yet, that strikes me as an *interesting* decision.
The L4 kernel is truly minimal. The Hurd depends on a couple of features in
Mach that simply don't exist in L4. The three core issues:
* Device drivers
* Virtual memory management
* A capability based IPC system
L4 does not have any device drivers. Some people are working on a new
device driver infrastructure for GNU Hurd/L4, and drivers for that
infrastructure. It's a huge task, but it is also an interesting challenge.
Device drivers will be in user space and thus can be multi-threaded. This
allows to keep state about a device active in the thread, and can greatly
simplify a driver.
L4 does not have any Virtual memory management, just some primitives to
recursively map pages into other address spaces. The physical memory is
provided directly to the user space. So we have to implement a VM system.
Neal has some great and ambitious ideas about doing VM management locally in
every task, instead of a global server. This is a dramatic departure from
traditional OS design (even from Mach and the Hurd as it is now), and thus
needs to be fleshed out from scratch (there are some academic papers on it,
but I don't know of a production system doing it this way).
L4 provides a minimal IPC system, that allows direct thread-to-thread
message sending. But it only gives a single protection primitive. So it is
not usable for a user space multi server system as the Hurd, where untrusted
communication is happen on a frequent basis. This is why a more featureful
capability system is needed. Instead doing it in a central global server
(like the port system in Mach is centralized), we want, again, do it locally
in each task. This seems to be a new design to me, and I have worked on the
protocols and data srtuctures for that in the last weeks. I am currently
working on an implementation, which is needed very early in any porting
effort to L4.
So you see, it is not about L4, but about glueing Hurd to L4. Mach did a
lot of things for us that L4 is not doing, and we take the chance to try to
more consequently implement the Hurd's fundamental design ideas. Freedom to
the users! Freedom to map your physical memory as you want it, freedom to
use the IPC policy you want, and the freedom to have functional device
drivers without getting them approved by a kernel master geek ;)
Thanks,
Marcus
--
`Rhubarb is no Egyptian god.' GNU http://www.gnu.org marcus@gnu.org
Marcus Brinkmann The Hurd http://www.gnu.org/software/hurd/
Marcus.Brinkmann@ruhr-uni-bochum.de
http://www.marcus-brinkmann.de/
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