l4-hackers February 2019

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

4 participants
4 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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Setting up ssd and hard disk in l4linux
by Xinyue Wu 26 Feb '19

26 Feb '19

Dear Developers, I have been trying to mount ssd and hard disk in l4linux. There are some similiar questions in previous archives, and I have done the following things according to them: - configure 'devs' file and 'io' file, add sata to vbus - enable PCI support - enable vPCI driver - enable AHCI driver But when I ran l4linux in hardware, I still couldn't find sata in pci bus, nor could I found my devices under /dev. Does anyone have some suggestions? Or maybe there are … [View More]

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some questions about l4bd.c in l4linux
by yadong.li 20 Feb '19

20 Feb '19

Hi Adam， I am studying how to support emmc storage in L4Re. I have read the file l4linux/drivers/block/l4bd.c. I have some questions 1、 In “l4linux/drivers/block/l4bd.c”, I can’t find the function implementation of “l4fdxc_request_write” and some with prefix l4fdxc in l4linx or l4， is there some demos? 2、 If it is implemented by ourselves, where is more reasonable to place “l4fdxc_request_write” , in l4 or l4linux ? 3、 If in l4, Which implementation is more reasonable , lib or … [View More]

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Communication between L4Linux tasks and L4re tasks
by Canberk Demirsoy 17 Feb '19

17 Feb '19

Dear Developers, I have a simple question regarding the communication of L4Linux and L4re. I have already read previous entries but could not solve my problem. I am able to establish a communication between the tasks in L4re, but what I need is communication between a task that is running on L4Linux and a task that is running on L4re. How can I do this in simplest way? Any suggestions? Best Regards *Canberk*

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l4-hackers February 2019