Category Archives: Networking

Network setup for a custom Qemu Virtual Machine

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After building a custom Qemu, there are a couple ways to run a VM to get to it. The older approach to VM management is to create a block device, run the VM with a boot device, do a full install and log in to the serial console. However, if you run the Qemu/KVM machine from the command lilne, hitting control C will stop your VM, and this is annoying. I have found it worth while to set up networking and then to SSH in to the machine.

My notes here suck. I am going to try and document what I have here working, and, over time, reverse engineer how I got here.

Edit: here are the steps

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A PCC driver in Qemu

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In order to perform test driven development, you need a way to drive your code that can isolate behavior. Linux Kernel drivers that communicate with hardware devices can be hard to test: you might not have access to the hardware from your test systems, or the hardware may be flakey. I have such a set of issues with the Platform Communication Channel (PCC) drivers I am working with.

My primary work has been with a network driver that only exists on the newest hardware. However, I also need to be able to handle some drivers that would only work against old hardware. There are also PCC based drivers for hardware that my company does not support or have access to. I might want to make a test to ensure that changes to the Linux Kernel PCC driver does not change its behavior against these drivers. There exists no system where all of these drivers would be supported. But I can build one with Qemu.

The Qemu based driver might not completely simulate the hardware exactly as implemented, and that is OK: I want to be able to do things with Qemu I cannot do with current hardware. For example, the MCTP-over-PCC driver should be able to handle a wide array of messages, but the hardware I have access to only supports a very limited subset of message types.

The full code for the device is here.

Here is how I went about building a Qemu based PCC driver.

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Viewing the Flattened Device Tree from Qemu

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The Qemu implementation uses a Flattened Device Tree (FTD) to manage the virtual implementation of the physical devices in a machine. I need to create a FTD entry for the MCTP-PCC implementation I am writing in Qemu. Since this is new to me, and I am working (as I most often do) via Ttrial and error, I want to see the FTD entry after I write it. Here is how I am dumping it.

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Debugging Qemu with gdb

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When developing Linux Kernel code, I have found myself wanting to have a test fixture inside the Firmware that lets me inspect the values communicated out of and into the Linux Kernel. I am currently writing one such fixture in Qemu. And I have an interrupt that is not getting handled by the Linux Kernel, I think because it is not getting delivered.

I have found it quite valuable to run this Qemu process in the Gnu Debugger. Here is how I (with help) got to the bottom of the mystery.

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Updated MCTP send code

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While the existing documentation is good, there are a couple things that have changed since it was originally written, and I had to make a couple minor adjustments to get it to work. Here’s the code to send a message. The receive part should work as originally published; what is important is the set of headers. I built and ran this on an AARCH64 platform running Fedora 38.

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Unifying Audio with Pipewire

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ALSA. Jack. PulseAudio. MIDI. Musescore. Jamulus.

My musical interactions with Linux are not the most complex in the world, but they ain’t trivial. The complexity of the Linux audio landscape has been a stumbling block so far. Pipewire has just gotten me past that.

The title of this article implies that you need to do something other than install Pipewire. So far, this is not true. On my system, at least, it Just works.

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Adding an IP address to a Bridge

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OpenShift requires a load balancer for providing access to the hosted applications. Although I can run a three node cluster, I need a fourth location to provide a load balancer that can then provide access to the cluster.

For my home lab set up, this means I want to run one on my bastion host….but it is already running HTTP and (FreeIPA) Red Hat IdM. I don’t want to break that. So, I want to add a second IP address to the bastion host, and have all of the existing services make use of the existing IP address. Only the new HA Proxy instance will use the new IP address.

This would be trivial for a simple Ethernet port, but I am using a Bridge, which makes it a touch trickier, but not terribly so.

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