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Kernel allocated memory not visible in L4Re virtual device despite correct physical RAM mapping
by stephen.yang 15 Apr '26

15 Apr '26

Hi l4-hackers, I am trying to pass a kernel-allocated memory buffer to a virtual device, but I am running into an issue. I would appreciate any help or suggestions. 1.Kernel side(driver) In my platform driver, I allocate memory using devm_kzalloc, write a string into it, and pass its physical address to the device via an MMIO register: …… void* buf = devm_kzalloc(&pdev->dev, 0x200, GFP_KERNEL); if (!buf) return -ENOMEM; phys_addr_t pa = virt_to_phys(buf); char *str = "hi stephen"; memcpy(buf, str, strlen(str) + 1); wmb(); writeq((u64)pa, hb->base + 0x00); …… My understanding is: allocate kernel memory → write data → pass physical address to the device. 2. Memory layout I observed that the allocated physical address falls within the RAM regions configured via bootargs : 0x66200000 - 0x6fffffff So my assumption was that if the virtual device maps this physical memory region, it should be able to access the data written by the kernel. 3. Virtual device side (L4Re) On the virtual device side, I map the physical memory using L4Re RM attach: void write(unsigned reg, char /*size*/, l4_uint64_t value, unsigned) { if (reg == 0x00) { addr = value; auto d = L4Re::Env::env()->get_cap<L4Re::Dataspace>("lram"); l4_addr_t vaddr; l4_addr_t base = 0x66200000; l4_addr_t size = 0x09E00000; int r = L4Re::Env::env()->rm()->attach( &vaddr, size, L4Re::Rm::F::Search_addr | L4Re::Rm::F::RW | L4Re::Rm::F::Cache_uncached, L4::Ipc::make_cap_rw(d), 0, L4_PAGESHIFT); if (r) { printf("can't attach hb memory\n"); return; } char *dst = reinterpret_cast<char *>(vaddr + (addr - base)); for (int i = 0; i < 20; i++) { printf("%02x ", (unsigned char)dst[i]); } printf("\n"); printf("get msg for guest: %s\n", dst); } } 4.Problem However, I cannot read the string written by the kernel. The memory content I get appears to be uninitialized or unrelated data. 5.Question I can confirm that the lram dataspace capability does correctly correspond to the physical memory region 0x66200000 - 0x6fffffff, and this region is valid and usable RAM on the system. I am unsure where the issue lies: (1) Is this approach (kernel allocation → pass physical address → direct mapping in virtual device) actually valid? (2) Could this be a cache coherence / DMA issue? (3) Or is there something incorrect in my L4Re RM attach usage? Any insights would be greatly appreciated.

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Mounting the Sd card on the L4re for Imx 95.
by Marka Sriharsha 08 Feb '26

08 Feb '26

The L4Re version I am using is r-2025-W48. Initially, I attempted to boot using the initramfs-based method. In this approach, I converted the uImage into a raw bin image, because loading the uImage directly resulted in a Bad kernel image error, likely due to the default uImage header handling. After flashing the converted binary to the SD card, the system booted but consistently got stuck during early boot. Even when different load addresses are specified in U-Boot, the kernel is always relocated to a fixed address (0x90080000). This address overlaps with a reserved memory region (e.g., vpu_boot@a0000000), leading to a region overlap error and reboot. I'm attracting the log (Initramfs.txt). After this, I switched to the SD card hardware passthrough method. Following the i.MX documentation, I prepared the SD card as follows: Partition 1: FAT partition containing bootstrap.uimage and the i.MX95 device tree Partition 2: Root filesystem As suggested, I updated the configuration (.cfg) file by following the tutorial, and this resolved the console misconfiguration issue. The system now boots correctly, and I am able to see the console output. For reference, I am attaching the boot logs(Boot_partition_2.txt) Although the system boots successfully, the SD card root partition is not being detected or mounted. While following the documented steps to enable SD card access, the boot process ends with a “no device found” error for the SD card. After further debugging, I noticed that for i.MX95, there is nohw_devices.iofile available in the following repository: [https://github.com/kernkonzept/io/tree/master/io/configs] Because of this, it is unclear how the SD/MMC hardware should be described for i.MX95, and the block device does not appear during boot. For reference, I am attaching the boot logs(Boot_partition_3.txt) Is there an official hw_devices.io configuration available for i.MX95? or without the hw_device.io also we can mount the sd card? Any guidance or example configuration would be very helpful for mounting the rootfs.

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Beyond the Bricks: How Google's Block Breaker Doodle Reminds Us of the Joy of Play
by blockbreakerapk10174＠gmail.com 06 Feb '26

06 Feb '26

In the relentless digital stream of our daily lives—a torrent of emails, notifications, and curated content—a moment of pure, unadulterated play can feel like a radical act. It was within this context that, on a seemingly ordinary day, millions of people worldwide were greeted by a delightful surprise on Google’s homepage: the Google Block Breaker Doodle. This wasn’t just a static logo; it was a fully playable, wonderfully crafted version of the classic arcade game, Breakout. With a click, the world’s most ubiquitous search portal transformed into a minimalist arcade, inviting users to smash a wall of colorful <a href="https://blockbreaker.ws/">google block breaker doodle</a> bricks with a bouncing pixel. The phenomenon was more than a nostalgic trip; it was a masterclass in design philosophy, a commentary on workplace culture, and a potent reminder of a fundamental human need. The genius of the Google Block Breaker Doodle lay in its elegant simplicity and its deep-rooted heritage. The original Breakout, released by Atari in 1976, is itself a piece of tech legend. It was a direct descendant of Pong, emphasizing solitary focus and rhythmic precision. More intriguingly, its original development involved a young Steve Jobs and Steve Wozniak, who were tasked with creating its circuit board. For Google to choose this game was a wink to its own Silicon Valley lineage, connecting the dots from the garage-built ethos of early computing to the sleek, global platform of today. The Doodle didn’t just reference a game; it honored a foundational moment in digital interactivity. But the impact went far beyond history. The user experience was perfectly calibrated. The mechanics were instantly understandable: move the paddle, bounce the ball, break the blocks. There were no tutorials, no loot boxes, no complex narratives. The objective was clear, the feedback immediate (the satisfying disappearance of a brick), and the failure state gentle—a chance to try again instantly. This created a state of "flow," that psychological condition of complete immersion and focused enjoyment. In a world of endless scrolling and multitasking, the Google Block Breaker Doodle offered a rare commodity: a single, simple task that commanded full attention. It was a digital mindfulness exercise disguised as a game. The timing and placement of the Doodle were equally significant. By situating this portal to play on the default homepage for countless browsers, Google shattered the traditional boundary between "work" and "play." For a brief moment, the tool we use for research, communication, and productivity became a site of leisure. This wasn’t an app you had to seek out; it was presented, almost insistently, as an alternative path. It encouraged what modern productivity gurus might call a "micro-break"—a short, intentional pause to reset the brain. The message was subtle but powerful: creativity and efficiency are not antithetical to play; they are often fueled by it. The Google Block Breaker Doodle became a sanctioned, global five-minute recess, challenging the grim grind of hustle culture. Furthermore, the Doodle served as a brilliant, unspoken showcase of web technology. This was no mere Java applet from the early 2000s. It was a smooth, responsive, and visually pleasing experience built with modern web standards, demonstrating what a browser could do without any plugins or downloads. It was a testament to the sophistication of accessible web design, proving that complex interactivity could be delivered instantly and seamlessly to anyone with an internet connection. In this way, the playful facade was also a technical flex, a demonstration of Google’s core competency in making the web a more dynamic canvas. On a societal level, the Doodle created a shared cultural moment. Social media lit up with high scores, strategies, and good-natured commiseration over near-misses. Colleagues compared scores by the watercooler. It democratized a specific joy, creating a common reference point across generations. Older users relived childhood memories in arcades or on early home consoles, while younger ones discovered a genre stripped bare of modern gaming’s complexities. The Google Block Breaker Doodle acted as a universal language, speaking through the simple verbs of aim, bounce, and break. Ultimately, the lasting lesson of the Google Block Breaker Doodle is about the enduring human need for purposeless purpose. We are wired to derive satisfaction from mastering simple systems, from seeing a clear cause and effect, and from engaging in activities where the only goal is the activity itself. In our increasingly optimized and outcome-driven lives, we often engineer this joy out of existence. The Doodle was a gentle intervention, a reminder that play is not frivolous. It is essential for cognitive maintenance, sparking joy, relieving stress, and fostering a creative mindset. Google, a company built on algorithms and data, used its most prized real estate to advocate for something beautifully analog in spirit: the click of a mouse, the bounce of a ball, the shattering of a digital wall. The Google Block Breaker Doodle was more than a clever homage; it was a manifesto in interactive form. It argued that within the cold architecture of code and connectivity, there must always be room for a paddle, a ball, and a wall of bricks waiting to be broken. It reminded us that sometimes, the most productive thing we can do is to simply play.

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VMs on L4Re
by Quang 16 Jan '26

16 Jan '26

Dear L4Re team, I'm following your guidance on the steps to get the RPi 4B running with L4Re (https://l4re.org/bsp/rpi.html) I can set up and run very well with your steps. However, I think an image, especially l4re_vm-multi_rpi4.elf, is only for 3 VMs. Could you help me figure out how I can increase the number of VMs, and each VM will run on a different OS, such as Ubuntu, Red Hat, or Kali Linux? Thank you for your support, and have a nice day. Best regards, Quang

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custom virtio
by Baruch Burstein 26 Dec '25

26 Dec '25

Hi all, I am looking into implementing a custom backend for a virtio console (I hope I am using the right terminology. The frontend will be in a VM managed by uvmm). I found 2 seemingly unrelated parts of the code named virtio-console, one under uvmm/server/src/virtio_console.[cc|h] and one under l4virtio/include/server/virtio-console. I am wondering why there are 2, what the difference is and when to use each one? There generally seem to be multiple things named virtio* in uvmm/server/src/, uvmm/server/src/device/, as well as separate repos like the aforementioned l4virtio or virtio-net. Thank you in advance for helping clear this up for me

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Multiple targets in one package
by Baruch Burstein 03 Dec '25

03 Dec '25

Hi. I am trying to build multiple apps in one package. I tried the following but am getting linker errors (of all things!) ``` PKGDIR ?= ../.. L4DIR ?= $(PKGDIR)/../.. TARGET = app1 SRC_CC = app1.cc common.cc REQUIRES_LIBS = libio cxx_libc_io cxx_io libstdc++ include $(L4DIR)/mk/prog.mk TARGET = app2 SRC_CC = app2.cc common.cc REQUIRES_LIBS = libio cxx_libc_io cxx_io libstdc++ include $(L4DIR)/mk/prog.mk ``` What is the right way to do this?

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[FOSDEM'26] Call for Participation: Microkernel and Component-Based OS Devroom
by Sebastian Sumpf 04 Nov '25

04 Nov '25

Hi there, here the CfP for FOSDEM'26: We are happy to announce the CfP for the "Microkernel and Component-Based OS" devroom at the upcoming FOSDEM 2026. Developers and users of various free and open-source microkernel-based and component-based operating systems will meet again at FOSDEM during the weekend of January 31st and February 1st 2026 and will share a developer room. The Microkernel and Component-Based OS devroom is currently looking for your participation in the form of proposals for talks, demos and other related activities. Possible topics for the devroom include, but are not limited to: * Introduction of a specific OS or framework * Design of subsystems and the general architecture of an OS * Languages, tools and toolchains used * Enabling support for hardware architectures, devices and programming languages * Development processes, debugging * Maintenance, testing and release engineering * Safety, security and robustness * Trends and challenges * Research and open questions * Community and governance * Use cases, experiences and status updates * Best practices and lessons learned * Live demos Important Dates The deadline for submitting proposals is December 1, 2025 (23:55 CET). Please use the FOSDEM website at https://fosdem.org/submit, select the track "Microkernel and Component-Based OS" and include at least the following information: * Title of your proposal * Short abstract (one or two paragraphs) * Duration (20 - 30 minutes, including demos and Q&A) * Your full name * Your short bio The devroom schedule (with accepted talks) will be announced on the devroom website at https://fosdem.org/2026/schedule/track/microkernel by December 15, 2025. The schedule will also be posted to the devroom mailing list and the speakers will be notified via email. Like previous years, the devroom will take place as an in-person event at the ULB Campus Solbosch in Brussels, Belgium during the second half of February 1, 2026 (Sunday). Due to the high number of submissions that FOSDEM received again this year, our devroom has only been allocated half a day. Therefore, the final decision on the duration of the talks will be made by the devroom organizers based on the number of accepted proposals. Given these restrictions, eight to nine half hour slots with a speaking time of 20 to 25 minutes and 5 to 10 minutes of Q&A form the upper limit. Requirements The communication language of the devroom is English. All content must relate to free and open-source software. By participating in the event, you agree to the publication of your talk recordings, slides and other content provided under the same CC-BY license as all other FOSDEM content. All participants are expected to adhere to the Code of Conduct: https://fosdem.org/2026/practical/conduct/ Contact For the 2026 edition, the organizers of the devroom are * Sebastian Sumpf (sebastian.sumpf [at] genode-labs.com) * Josef Söntgen (josef.soentgen [at] genode-labs.com) For any comments, questions and suggestions, please do not hesitate to contact us directly or via microkernel-devroom-manager at fosdem.org. Announcements and inquiries are additionally available through the devroom mailing list https://lists.fosdem.org/listinfo/microkernel-devroom About the Devroom Since the first Microkernel OS Devroom at FOSDEM 2012, this devroom has been a part of each subsequent FOSDEM (with slight variations of the name). The focus gradually widened to include component-based and other operating systems. It has now become somewhat of a tradition for the open-source operating systems community, a kind of "extended family meeting" and it is one of the very few places where microkernel and operating-system enthusiasts meet regularly. To date, over two dozen projects have participated in one way or another. Many of the projects face similar challenges but come up with partially different solutions. Therefore, the goal of the devroom is to bring the various projects together, let them exchange ideas, cross-pollinate and socialize. As in the past, the devroom will offer the stage both to established community members and to newcomers. Our goal is to select talks in an inclusive manner and to create a balanced and unbiased schedule, while keeping the quality of the devroom as high as in previous years. About FOSDEM FOSDEM is a two-day conference organized by volunteers to promote the widespread use of free and open-source software. FOSDEM is widely recognized as one of the largest and best such events worldwide. FOSDEM covers a wide spectrum of free and open-source software and hardware projects and offers a platform for people to collaborate. To this end, FOSDEM has set up developer rooms (devrooms) where teams can meet and showcase their projects. Devrooms are a place for teams to discuss, hack and publicly present new ideas, latest results, lightning talks, news and proposals. Besides developer rooms, FOSDEM also offers main tracks, lightning talks, certification exams and project stands. In recent years, FOSDEM has been hosting more than 5000 developers annually at the ULB Solbosch Campus in Brussels, Belgium. The participation in FOSDEM is totally free and requires no registration, although the organizers gratefully accept donations and sponsorship. Devroom Dinner Over the years, it has become a tradition that devroom speakers and enthusiasts meet for dinner somewhere in Brussels after the event to continue discussions, socialize and meet old and new friends. We plan to follow that tradition in some form as well in 2026 and will inform you about the exact arrangements later. Hope to see all of you in Brussels! Regards, Sebastian -- Sebastian Sumpf 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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Sculpt 25.10 with Fiasco.OC kernel
by Alexander Boettcher 03 Nov '25

03 Nov '25

Hello, last week Genode released a new version of its open source OS - Sculpt OS 25.10 [0]. I took the opportunity to craft a PC image that also contains an (unofficial and experimental) Sculpt variant based upon a Fiasco.OC kernel [1] version. The variant reflects to large degree the state I presented live during the Systems Meetup [2] gathering at the TU Dresden in January 2025. In case you are interested, feel free to check out [1]. Cheers, Alex. [0] https://genode.org/download/sculpt [1] https://genodians.org/alex-ab/2025-11-02-sculpt-multi-kernel [2] https://ukvly.org -- Alexander Boettcher Genode Labs https://www.genodians.org - https://www.genode.org

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Assertion ‘state() & Thread_vcpu_user' in kernel vgic interrupt processing
by yy18513676366 14 Oct '25

14 Oct '25

Hi l4-hackers, When running a virtual machine, I encounter an assertion failure after the VM has been up for some time. The kernel crashes in src/kern/arm/thread-arm-hyp.cpp, specifically in the function vcpu_vgic_upcall(unsigned virq): vcpu_vgic_upcall(unsigned virq) { ...... assert(state() & Thread_vcpu_user); ...... } Based on source code inspection and preliminary debugging, the problem seems to be related to the management of the Thread_vcpu_user state. （1）Under normal circumstances, the vcpu_resume path (transitioning from the kernel back to the guest OS) updates the vCPU state to include Thread_vcpu_user. However, if an interrupt is delivered during this transition while the receiving side is not yet ready, the vCPU frequently return to the kernel (via vcpu_return_to_kernel) and subsequently process the interrupt through guest_irq in vcpu_entries. In this situation, the expected update of Thread_vcpu_user may not yet have taken place, which seems result in the assert being triggered when a VGIC interrupt is involved. （2）A similar condition seems to occur in the vcpu_async_ipc path. At the end of IPC handling, this function explicitly clears the Thread_vcpu_user flag. If a VGIC interrupt is delivered during this phase, the absence of the expected Thread_vcpu_user state seems to lead to the same assertion failure. I would like to confirm if the two points above are correct, and what steps I should take next to further debug this issue. In addition, I have some assumptions I would like to confirm: First, for IPC between non-vcpu threads, the L4 microkernel handles message delivery and scheduling (wake/schedule) directly, without requiring any forwarding through uvmm. Similarly, interrupts bound via the interrupt controller (ICU) to a non-vcpu thread or handler are also managed by the kernel and scheduler, and therefore do not necessarily involve uvmm. Second, passthrough interrupts, when not delivered in direct-injection mode, are routed to uvmm for handling if they are bound to a vCPU. Likewise, services provided by uvmm (such as virq) are also bound to a vCPU and therefore require forwarding through uvmm. There seems to have been a similar question in the past, but it does not seem to have been resolved. Re: Assertion failure error in kernel vgic interrupt processing - l4-hackers - OS Site I wonder if my questions are related to that post, and if any solutions exist. Thanks!

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Questions regarding L4::Platform_Control
by David Q 13 Oct '25

13 Oct '25

Hello L4 hackers, I would like to thank the community for helping me learn; I am back with more questions. This time it is about L4::Platform_Control 1. It seems IO and UVMM both have this L4::Platform_Control capability, is it correct? 2. I can't seem to create a usable L4::Platform_Control capability in a userland application using the factory from env. Does this mean there is one such platform control capability only? 3. It seems there is a system_suspend() function, and X86 is particularly supported. Does it work right now for ARM? 4. Once system_suspend() is called in an ARM environment, does the kernel communicate with PSCI, which then gets intercepted by Arm trusted firmware? 5. Also, before suspending the entire system, it would seem a good idea to quiesce all threads (except the kernel I assume) and save the CPU/Thread states/contexts. Is there an API that could help with quiescing threads and saving relevant states? I only see the debugger being able to quiesce all threads, but I can't seem to find an API that can save and reload the saved states once we want to exit suspend. These are a lot of question, and I thank anyone who reads them.

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