Hi Norman,
Thanks for the detailed suggestions with so much patience. I'm a practitioner, so I really appreciate details, and I'll check out the stuff that you've mentioned and try to get started ASAP.
BTW, do you have any suggestions on small related tasks that I can take in prior to my formal application for the GSoC project? I think committing small patches would be a great starting point to getting familiar with the Genode codebase.
Best Regards,
Zhongze Liu.
2017-03-01 19:37 GMT+08:00 Norman Feske <norman.feske@...1...>:
Hello Sky Liu,
thank you for your interest in Genode!
I'm a college student majoring Information Security in HUST, and I'm interested in genode's GSoC project on microkernelizaing the Linux kernel. I've been quite interested in microkernel OSs and virtualization, with limited experiences working on xen and the linux kernel. and have been in the MINIX community for a short time, so I was attracted by this challenge at the first sight.
However, I'm still new to the genode project. So I'll appreciate it if I could get from you some suggestions on where to get started. :)
The best way to start exploring Genode is the "Genode Foundations" book, which you can download here:
http://genode.org/documentation/genode-foundations-16-05.pdf
I recommend you to at follow the getting-started section, and skim over the Chapter 3 (Architecture) and Section 4.7 (Component compositions) to get a tangible feeling for Genode.
To practically tackle the "microkernelization of Linux", there are actually two approaches. (1) The first approach is to enable Genode to access devices of the Linux system you are working on. This is convenient, but it bears the risk that a device driver (running in a Genode component) may interfere with the Linux kernel, crashing the system. The other approach (2) is booting a custom-made Linux system + Genode's core as init process in Qemu. The latter approach would be equivalent to how we work with the various microkernels.
Depending on your interests, you may quickly dive in into the actual Genode code (1) or work on a custom run environment for Linux-based Genode system (2) first.
Regarding the actual topic, the overall challenge is allowing Genode's device drivers access to the hardware that is normally accessed by the Linux kernel only. From Linux' point of view, Genode appears as a user-level device driver. So one piece of the puzzle is to gain a good understanding of Linux' user-level device-driver support. From Genode's perspective, device hardware is accessed through the IO_MEM, IO_PORT, and IRQ services of Genode's core component. So in principle, the topic comes down to implementing these services for the 'base-linux' version of core by using Linux' interfaces for user-level device drivers.
There are several stages:
By implementing the IO_PORT service, Genode components can interact with simple port-I/O-devices such as the PIT timer. The implementation should by straight-forward: When running core at I/O privilege level (IOPL) 3, core can execute the regular 'inb', 'outb', etc. instructions. A simple test component could request a Genode IO_PORT session for, let's say, the PIT, program the PIT as a running counter, and repeatedly read the current counter value.
Non-trivial devices need access to memory-mapped I/O registers. Core's IO_MEM service makes such registers available to its clients as a dataspace (the concept is explained in the book). On Linux, dataspaces are represented as memory-mapped files, passed from core to the driver component by passing a file descriptor, and attached to the driver's address space via 'mmap'. Consequently, the problem comes down to core obtaining a file descriptor for a given physical address region. Here the challenge is to find a suitable Linux kernel mechanism that hands out portions of physical memory as a file descriptor.
With the principle support for memory-mapped I/O and port I/O in place, it should be possible to run the VESA framebuffer driver.
Most drivers use device interrupts. To use those drivers, core's IRQ service needs to be implemented. Again, this calls for an investigation of Linux' user-level device driver support.
Direct memory access. Many drivers use DMA to let the device write directly into memory. For this to work, the driver must supply the targeted memory address to the device. On systems without IOMMU, this is the physical bus address. In order to use DMA on Genode/Linux, DMA buffers need to be allocated as contiguous physical memory and their physical addresses must become known to the user-level driver component. In systems with IOMMU, the device-physical addresses a virtualized. So there is more freedom. But the details ultimately depends on the Linux handling of IOMMUs.
In Genode, PCI devices are managed by the so-called platform driver. When a regular device driver needs access to a certain device, it does not use core's IO_MEM, IO_PORT, and IRQ services directly but it requests a platform session. A platform session is like a virtual bus where one or multiple devices are present, depending on the platform driver's policy. The platform session makes the device resources of those devices available to the client (the device driver). Under the hood, the platform driver opens IO_MEM/IO_PORT/IRQ sessions at core.
In order to use Genode's existing arsenal of device drivers on Linux, we need either a platform-driver version specifically adapted for Linux (when re-using the Linux PCI driver), or investigate a way to use our regular platform driver (including the PCI driver) directly (this would be pretty cool!).
I hope that the level of detail does not frighten you. We certainly don't expect you to complete all these stages. E.g., if completing stage 1 to 3, there would already be demonstratable results. Stage 4 certainly requires an investigation into the ways about the interplay of the IOMMU handling of the kernel with user-level device drivers. Stage 5 also has a atrong design aspect to it.
It goes without saying that we won't leave you on your own. :-)
Cheers Norman
-- Dr.-Ing. Norman Feske Genode Labs
http://www.genode-labs.com · http://genode.org
Genode Labs GmbH · Amtsgericht Dresden · HRB 28424 · Sitz Dresden Geschäftsführer: Dr.-Ing. Norman Feske, Christian Helmuth
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