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February 2020

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L4Re with GCC 9; Repositories and Roadmaps
by Paul Boddie 06 Feb '20

06 Feb '20

Hello and Happy New Year! Having been otherwise occupied so far this year, I decided to take the opportunity to revisit my L4 development environment that happens to use Debian unstable. It turns out that GCC 9 is now the supported toolchain version, and unfortunately, L4Re doesn't yet understand this new version. So, first of all, I wondered whether support for that is imminent. I did take a look at persuading the build system to work, but this seems to involve hunting down version-specific resources and defining new ones for the new version. I stopped when encountering some libstdc++ headers and wondering whether this isn't documented somewhere. = Repositories = It did occur to me that I might also be expecting updates for such things from the wrong channel. Since I have been using L4Re for a while, I still use the Subversion repositories made available from svn.l4re.org, referenced by the documentation on l4re.org. It seems that these repositories are not updated as frequently as the GitHub repositories, but maybe they serve as a kind of staging area for more stable releases. After giving up on using GCC 9, I decided to re-clone repositories in a Debian stable environment. This didn't go very well with the Subversion repositories: after a while of saturating my aging machine's I/O bandwidth, cloning (or checking out) tended to fail. The repomgr script, if re-run, seems to just delete everything and start again, so after a few tries I gave up. Meanwhile, using the GitHub repositories failed for different reasons. Previously, I have tended to get "index pack" and "missing object" errors with Git when cloning. There don't seem to be any decent remedies for this, with people suggesting tweaking compression, network, memory and other settings, manually fiddling with the "object database" and other such nonsense. The consensus might be that I do not have a powerful enough machine (only 1GB RAM) to clone even modestly-sized Git repositories. Nevertheless, it would be nice to be able to download some kind of release distribution. I somehow doubt that the world really needs to accommodate all the multi-gigabyte Linux kernel clones out there that differ by a few files, either. If I really want to version my modifications against the released code, I can always version control them myself (even using a more coherent version control system if I want). = Roadmaps = Such considerations led me to also consider what I have been wanting to use L4Re for and the notion of a roadmap, both my own and that of L4Re itself. It seems to me that L4Re, or parts of it, can form the basis of a usable system in its own right (as opposed to just hosting L4Linux). However, I don't see much development directed towards such a goal. (Actually, I don't really see much L4Re development at all.) It seems to me, as I may have mentioned before, that one crucial area of development that would be needed for my purposes is that of providing general- purpose filesystem support. To do that, it seems that a few separate issues need attention, amongst them the component architecture required to export file entries and contents to programs, and support in the C library so that programs can access files transparently. Since L4Re's filesystem support seems to require linking filesystem-specific libraries against programs and having those programs access filesystem data directly, as far as I can tell, a new component architecture is required. Meanwhile, it seems to me that uClibc, at least in the version offered by L4Re, does not offer a particularly coherent or convenient basis for integrating with such a component architecture. Consequently, I spent a fair amount of time last year investigating filesystem component architectures and alternative C library implementations. But I wouldn't mind knowing whether there are plans to remedy these deficiencies in L4Re or whether I should just plug away at such things by myself. (I can live with people saying that they don't care about the things that I happen to care about. No-one needs to make excuses here: even the Hurd people seem pretty unmotivated about getting people on board, although it can be said that within certain parameters, they at least have a system that meets their objectives to some degree.) = Some Thoughts = It rather looks like the amount of time I have to spend on L4Re-related development will be significantly reduced for the foreseeable future. In some respects, I feel that I have learned quite a bit, but in other respects, I feel that it has been a frustrating exercise to try and get up to speed with L4Re development. (And I hardly want to get started on my Fiasco development experience.) Although literature obviously exists about various L4 implementations, I don't feel that the documentation experience is coherent at all. Many of the pertinent details are also obscured by the focus on code-originating reference documentation. And where microkernel-based systems should be emphasising the nature of the interfaces between components, there instead seems to be a policy of hiding such details in the implementation details of various abstractions (implemented multiple times in an almost archaelogical fashion). Indeed, this latter policy is quite infuriating because it binds the developer to existing functionality and frameworks unnecessarily. So, when developing my components for low-end devices, I went along with L4Re framework classes due to an incomplete understanding of the IPC mechanisms (which also brought other irritations because those classes assume a more complete understanding), but also because interfacing with other components is not obvious when the protocols are squirreled away across multiple header and implementation files. I have even gone as far as to write my own interface description language tool to permit the generation of IPC code and to help emphasise component interfaces and protocols. It seems to me that a lot of the point in having systems that facilitate component-oriented architectures is to make such details explicit, as was indeed emphasised by projects like Spring. Otherwise, everyone might as well be writing stuff for frameworks in the Linux kernel. Anyway, I am sure that there are plenty of people developing for and enhancing L4Re who are content with their situation and what they are able to achieve. But I personally feel that opportunities are being missed to apply these technologies to areas where they would make a real (and, crucially, positive) difference. If anyone else (still reading by this point) is even vaguely interested in general-purpose operating system development on L4Re, it would be nice to hear from you. Paul

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