October 3rd, 2013


virgil3d: an update

Okay its been a while, so where is virgil3d up to now I hear you ask?

Initially I wrote a qemu device and a set of guest kernel drivers in order to construct a research platform on which to investigate and develop the virgil protocol, renderer and guest mesa drivers based on Gallium3D and TGSI. Once I got the 3D renderer and guest driver talking I mostly left the pile of hacks in qemu and kernel alone. So with this in mind I've split development into two streams moving forward:

1) the virgil3d renderer and 3D development:
This is about keeping development of the renderer and guest driver continuing, getting piglit tests passing and apps running. I've been mostly focused on this so far, and there has been some big issues to solve that have taken a lot of the time, but as of today I got xonotic to play inside the VM, and I've gotten the weston compositor to render the right way up. Along with passing ~5100/5400 piglit gpu.tests.

The biggest issues in the renderer development have been
a) viewport setup - gallium and OpenGL have different viewport directions, and you can see lots of info on Y=0=TOP and Y=0=BOTTOM in the mesa state tracker, essentially this was more than my feeble brain could process so I spent 2 days with a whiteboard, and I think I solved it. This also has interactions with GL extensions like GL_ARB_fragment_coord_conventions, and FBOs vs standard GL backbuffer rendering.

b) Conditional rendering - due to the way the GL interface for this extension works I had to revisit my assumption that the renderer could be done with a single GL context, I had to rewrite things to use a GL context per guest context in order to give conditional rendering any chance of working. The main problem was using multiple GL queries for one guest query didn't work at all with the cond rendering interface provided by GL.

c) point sprites - these involved doing shader rewrites to stick gl_PointCoord in the right places, messy, but the renderer now has shader variants, however it needs some better reference counting and probably leaks like a sieve for long running contexts.

2) a new virtio-gpu device

The plan is to create a simple virtio based GPU, that can layer onto a PCI device like the other virtio devices, along with another layer for a virtio-vga device. This virtio based gpu would provide a simple indirect multi-headed modesetting interface for use by any qemu guests, and allow the guest to upload/download data from the host side scanouts. The idea would be then to give this device capabilities that the host can enable when it detects the 3d renderer is available and qemu is started correctly. So then the guest can use the virtio gpu as a simple GPU with no 3D, then when things are ready the capability is signalled and it can enable 3D. This seems like the best upstreaming plan for this work, and I've written the guts of it.

In order to test the virtio-gpu stuff I've had to start looking at porting qemu to SDL 2.0 as SDL 1.2 can't do multi-window and can't do argb cursors, but SDL 2.0 can. So I'm hoping with SDL 2.0 and virtio-gpu you can have multiple outputs per the vgpu show up in multiple SDL windows.

I'll be speaking about virgil3d at the KVM Forum in Edinburgh in a couple of weeks and also be attending Kernel Summit.