This is the third post of a walkthrough of new device emulator development for QEMU which uses igb, a recently introduced Intel NIC emulation as an example. The first post roughly described there are several development steps:

  1. Writing a boilerplate and adding basic features
  2. Adding QTest
  3. Adding advanced features and libvirt support
  4. Running more tests and increasing feature coverage
  5. Debugging
  6. Contributing to the upstream

This post discusses 3 and 4. The goal is to make the device implementation, which was introduced in the previous post, usable for practical purposes.

Adding advanced features and libvirt support

Determining test workloads

The previous post discussed bringing up the first device implementation and letting the guest initialize the device. In the case of igb, it is not very difficult to bring features implemented for e1000e, the predecessor in Intel’s NIC product line, to igb as these devices are very similar, and igb development started by forking e1000e’s code. However, this initial state of the device emulation is not really usable; you would expect the new device will bring features not present in the existing devices.

Therefore, the next step of the device development will be to implement such advanced features. As described in the first post, the most notable features of igb are VMDq and SR-IOV. VMDq implements L2 switching among (nested) VMs, and SR-IOV allows to attach some tx/rx queues as a virtual function to a VM. The first thing to do when implementing a new feature is to fabricate a workload that will exercise it. To exercise VMDq and SR-IOV features, we have to perform the following operations on a Linux guest:

  • Enabling SR-IOV virtual functions
  • Sending and receiving packets with different combinations of peers:
    • The physical function and an external host
    • The physical function and a virtual function
    • A virtual function and an external host
    • A virtual function and the physical function
    • A virtual function and another virtual function

The physical function is the PCI function always available. Testing the various combinations of peers is important to ensure the L2 switching capability VMDq provides correctly functions.

Once the workloads were determined, you can test it and implement missing features and fix bugs for them, just like the basic features are tested and fixed. This process is incremental so it is important to prevent regressions and to ensure forward progress. It is better to continuously run qtest and simple workloads like ping and curl. Commit changes to Git frequently so that you can isolate regressions by e.g., running git bisect.

Note that it is not necessary to complete this step before sending the new device implementation to the upstream. It is actually often better to send the patches upstream as soon as you get confidence in the implementation of the basic functionalities. A small patch that implements the basics is easier to review than a long patch that includes many advanced features. The upstream reviewers can also discuss the design of the implementation early and you can correct it before writing more changes if you send patches early. It is even possible to send patches before you thoroughly test them as RFC.

For igb, I sent the first version of patches when VMDq and SR-IOV features become usable on Linux. The other target platforms, Windows and DPDK followed later.

libvirt support

libvirt is a very popular frontend for QEMU so we need it to have the support for the new device to some extent at least. The details of the code change vary by the type of the device, but the principle same with QEMU applies: find a similar device, copy the code for it, and extend it if necessary.

libvirt often requires few changes for a new device. The libvirt change for igb is just a copy of e1000e support, and you can see it at:

Running more tests and increasing feature coverage

As you add more and more features, it gets harder to continuously test all of the implemented features. Automated testing is essential to keep various features working.

It is not common that there are tests for the emulated device as hardware vendors do not release tests they presumably created during the hardware development. It is theoretically possible to develop a complete unit testing suite that covers all the features, but in practice, it consumes too much time.

A more practical way to introduce automated testing is to borrow existing test suites for drivers and platforms. As they target more high-level constructs, sometimes they do not suit well for device testing. For example, such a test suite may include tests for a pure software construct, which costs time and contributes nothing to device development. Such a test suite neither tests the device functionalities the platform does not use. Nevertheless, high-level tests are pragmatically very useful as they can ensure the success of the ultimate objective: to get the device to work for the target platform.

Usually, there are test suites specialized for a device type and an operating system so you have to choose appropriate ones. Such test suites for NICs and popular platforms will be presented below:


ethtool is a Linux utility for controlling network devices. The kernel exposes an ioctl that triggers device tests when requested by ethtool. The coverage of the tests is fairly limited but it is so easy to use that it is automated for igb using Avocado testing framework. The automating script is available at:

Linux Test Project (LTP)

Its README describes itself as follows:

The LTP testsuite contains a collection of tools for testing the Linux kernel and related features.

As the README states, LTP is not specific to networking and contains various tests for Linux. The network tests have their own README.

The main advantage of LTP is that it contains various kinds of tests from ping to applications like HTTP, FTP, telnet, etc. The downside is that its network tests are somewhat unmaintained, and require manual setup of network applications.

I have sent patches to fix the tests for a modern system, and wrote some scripts to manage VMs. These scripts are particularly helpful as they allow us to easily reproduce the model system for manual testing/debugging and to automatically run the LTP. The automated scripts test all of the combinations of peers described above.

As Linux was the first target platform, the LTP was the first test suite igb passes. However, later changes for the other platforms sometimes regressed the Linux support and automated LTP runs greatly helped to spot such regressions.

Windows Hardware Lab Kit (HLK)

The Windows Hardware Lab Kit (Windows HLK) is a test framework used to test hardware devices and drivers for Windows 11, Windows 10 and all versions of Windows Server starting with Windows Server 2016.

Windows Hardware Lab Kit | Microsoft Learn

Windows HLK includes comprehensive tests for network devices and exercises more advanced features like VMDq and SR-IOV. We maintain AutoHCK, a tool for automating HLK runs on QEMU so it is quite easy to set up VMs using it. Windows HLK spotted several bugs which were missed by the LTP in the case of igb.

The downside of Windows HLK, or rather Windows platform is that it is closed-source. When I first tried igb with Windows, its driver just hung and repeated resets without any helpful information. You can only see traces from QEMU and the disassembly of the driver. The next post will discuss debugging with this closed-source platform, but you will want to get the device to work with an open-source platform before testing with Windows.

DPDK Test Suite (DTS)

DPDK is not an operating system like Linux and Windows, but an application framework for low-level packet processing. DPDK implements its own driver to implement the interaction with the hardware in the userspace and to eliminate context switches with the kernel. The DPDK driver needs to be tested as it employs low-level hardware access as operating system drivers do.

The nice thing with DPDK is that it can be debugged as a normal user-space application. It can also use more minor device features, and DPDK Test Suite covers details of such features. However, DPDK is not for a user-facing application like curl so DTS cannot ensure the usability of the device emulation in common situations.


This post discussed how to add advanced features and libvirt support, and to test them. This part of device development takes a significantly long time so you need some strategy. You have to choose features and tests appropriate for the current development stage and sometimes send (RFC) patches to get design reviews.

The development process is not always smooth, and you may have difficulty getting a new feature to function, or the device implementation may suffer from regressions. Therefore the next post will mostly be spent on one thing: debugging.