l4-hackers March 2019

l4-hackers@os.inf.tu-dresden.de

5 participants
5 discussions

Filesystem Architectures with L4Re and L4-Based Systems
by Paul Boddie 02 Dec '19

02 Dec '19

Hello, As this list can attest, I previously spent a bit of time diving into the way dynamic linking is supported by L4Re to get shared mode programs working. This turned out to have a relatively simple fix related to symbols employed by GCC- generated code. But during this exercise, I gained some familiarity with the way libraries are loaded, and this appears to involve the virtual filesystem support, with the loader perusing the filesystem to access libraries, and with these being … [View More]provided as modules by the "rom" filesystem. Since then, I've spent some time looking at how files are provided by filesystems and accessed by code that uses the conventional C or POSIX library interfaces. It raises a few questions about why things are done in a particular way in L4Re, and which approaches are in use for other L4-based systems, especially those that seek to provide general-purpose, multi-user solutions. What I think I now understand about the virtual filesystem support in L4Re is as follows. The virtual filesystem itself is a "client-side" construct, meaning that it resides within any given program. That programs have their own view of a filesystem is not too different from what the documentation about systems like the Hurd describes and advocates, and I suppose it is a tempting approach because it gives each program the flexibility to be customised in this regard. However, it seems that beyond the internal "mount tree" representation, configuration of the namespace hierarchy is largely done using the L4Re namespace concept, with namespaces acting as directories, and non-namespace objects within the hierarchy interpreted as particular file types. Apart from the interpretation of namespaces as directories, it seems that for the most part, dataspaces are employed to be interpreted as file objects. I will admit that I didn't really look very hard at how different systems provide file access before now, but what surprised me slightly was the apparent lack of delegation in L4Re. As far as I can tell, on systems based on kernels like Linux, the open library function will employ a system call to delegate the matter of finding the file and obtaining a way of accessing it to the familiar monolithic-kernel-plus-filesystems arrangement. On more "exotic" systems (than Linux) like Minix 3 or the Hurd, it appears that delegation to a virtual filesystem server [1] or servers [2] occurs to mediate access to specific files. Some systems like Inferno and Plan 9 employ protocols [3] to formalise the client-server relationship. I had almost expected to find similar things in L4Re, but I could only confirm their absence after reviewing the various VFS abstractions. One thing I wonder about is whether the current L4Re approach is able to satisfactorily deal with filesystem content that needs to serve different user entities, with content having different permission and ownership rights. It seems that the filesystem logic has to be available as a library within a program, which is not necessarily a problem. But to avoid filesystem content being completely exposed to the discretion of user programs (reminiscent of the problems with early Network File System implementations), the library would need to call out to other entities, which doesn't appear to be done with the current L4Re abstractions. One could implement support for, say, the ext3 filesystem in a library, operating on a dataspace provided at some kind of mountpoint in a directory (with the directory being provided by a namespace, of course) but it would be like giving a user program access to a block device in a traditional Unix-like system. Are there any articles about the design of L4Re that might explain the motivations here? And are there any accessible-but-detailed articles about other microkernel-based systems and the way in which they structure their filesystem architectures? I looked around on the TU-Dresden site for materials, but I didn't immediately find anything obviously relevant. Paul [1] https://wiki.minix3.org/doku.php?id=developersguide:vfsinternals [2] https://www.gnu.org/software/hurd/hurd-talk.html#pat [3] http://doc.cat-v.org/inferno/4th_edition/styx [View Less]

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L4Linux with L4Re and Fiasco.OC-UX
by Paul Boddie 04 Apr '19

04 Apr '19

Hello, I have been looking at trying out L4Linux in UX mode as a first step towards its use on "native" Fiasco, and I seem to be experiencing a problem with launching the Linux kernel itself. I have followed the instructions from these pages: https://l4linux.org/download.shtml https://l4linux.org/build.shtml https://l4linux.org/use.shtml Specifically, I have an existing build of UX that has been used for some time to develop software. I have updated it to revision 83 and have checked … [View More]

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Raspberry Pi 3B and L4Linux
by Dejan Cotra 27 Mar '19

27 Mar '19

Hi all, Im trying to run L4Linux on Raspberry Pi 3B. So far I was able to build Fiasco kernel and L4Re for Raspberry Pi 3B. I used aarch64-linux-gnu-gcc toolchain. Also I was able to create Uboot image for hello config, which I successfully run on both qemu and Raspberry Pi 3B. So now Im trying to build L4Linux. First question would be does L4Linux supparm64 architecture? If yes, any hints for configuration? If it does not support arm64 architecture does this also mean that I have to build … [View More]

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Invalidating mapped flexpages in pagers
by Paul Boddie 19 Mar '19

19 Mar '19

Hello, I have been trying to implement a pager in L4Re where the pager employs a memory region of limited size to satisfy map requests for a dataspace occupying a larger virtual memory region. Upon receiving a map request, the pager identifies the dimensions of the flexpage to be sent to the task experiencing the page fault, and it populates the flexpage's memory with the appropriate data. What this accomplishes so far is that when page faults occur in a task accessing the dataspace, … [View More]

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Huawei R&D lab in Dresden
by Martin Decky 07 Mar '19

07 Mar '19

[Apology for this job ad. Please ignore if you are not interested in job offers.] Dear all, Huawei is starting a new R&D lab in Dresden. The lab shall focus on microkernel OS design and development, with a well-balanced mix between innovative research (including, but not limited to, topics such as HW/SW co-design, heterogeneous computing, data-centric computing, etc.) and prototype development (including, but not limited to, design and implementation of key operating system services for … [View More]

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l4-hackers March 2019