Category Archives: C

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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Stacking Protocols

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I find myself writing a program in C that is supposed to handle multiple protocols. At its entry point, the protocol is Platform Communication Channel (extended memory, type 3 and type 4). Embedded in that is an Management Component Transport Protocol (MCTP) message, and embedded in that is one of many different protocols.

I might want to swap out the PCC layer in the future for….something else. MCTP can come over many different protocols, so there is a good be that the tool will be more useful if it can assume that the protocol outside of the MCTP layer is something other than MCTP.

One problem I have is that the MCTP header does not have a length field. We do not not know how long the payload is; all it has is version, source, destination, and flags. Thus, if we want to pass a buffer of type MCTP header along, and we want the length, we need to pass it in a separate field. This goes both for incoming (how many bytes to read) and outgoing (how many bytes to write).

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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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