I’ve been working on a bunch of things across the Linux puzzle for a product I want to build. Here is an overview of a few of those things.

LibMKS at 120hz

I wanted to get my virtual machines to 120hz so that I can start testing product features inside of VMs. In fact, I actually like doing development with virtual machines over say, trying to shove all your development tooling in a sysext which, at least to me, feels like square-peg/round-hole territory.

To get this working, a few things needed improvements.

Qemu

Qemu has a dbus display backend where it can send you DMABUF FD. But it doesn’t really handle any sort of sync and that becomes a problem as you crank up the frame rate. Additionally, it just defaulted to 75hz with no mechanism to override it.

So I have some patches which provide a new D-Bus interface which can be implemented by LibMKS. It provides something more like a Vulkan swap-chain as well as API to set the refresh rate. While this isn’t a mapping 1:1 of what a wayland protocol might do for frame rate, it does match more what the emulated graphics device expects, so it is probably fine for now and clearly an huge improvement.

A big change in the new API is that we will register all the DMABUF up front, and then tell the client just to switch to another DMABUF along with damage rectangles. Of course, I also had to make Qemu start collecting damage rectangles correctly.

• DMABUF3 Listener API for D-Bus
• Allow Clients to set Frame Rate

Linux

With those changes in place, I kept seeing damage being full-frame. The next part of the stack that can break is thus the Linux kernel virtio graphics driver. Damage rectangles come in as properties on the drm plane being submitted. So it turns out that in two places some short circuiting was preventing that from working right.

After fixing all that (and the corresponding LibMKS side) I have decent graphics performance in a VM.

Since I continue to be floating precariously abroad, this is my notice of such patches. If you are interested in seeing these upstream and work in either of those communities, feel free to crib them, improve them, and submit them upstream. I’m happy locally patching my software given the copious amount of free time I have so there is little incentive for me to collaborate with corporations.

• Virtio fix 1
• Virtio Fix 2

Combined with the LibMKS merge request !53 I can have both minimal damage rectangles all the way to host GPU scanout as well as drag windows around in the guest quite fast.