Several months ago started the GSoC 2017. Among all the projects available one got my attention:
In a VM, to access real hardware, we have two methods: passthrough, and virtualization extensions (Intel VT-x, AMD-V..). When it comes to GPUs possibilities drop down to one: passtrough. Intel has a virtualization extension (GVT), but we want to support every devices. Thus, we need to fall-back to a software based method.
- Emulation ? Since we want 3D acceleration, better forget it
- API-forwarding ? This means we need to have the same OpenGL API between guest host, also no.
- Paravirtualization ? Yes !
Since a couple years, VirtIO devices became a good standard on QEMU. Then, Dave Airlie started to work on VirGL and a VirtIO-gpu. Both help provide a descent virtual-GPU which rely on the host graphic stack.
This article will present VirtIO devices, and what kind of operations a guest can do using VirGL.
I also invite you to read a previous article I wrote about Linux’s graphic stack
Since we will use a VirtIO based device, let’s see how it works. First, these devices behave as regular PCI devices. We have a config space, some dedicated memory, and interruptions. Second very important point, VirtIO devices communicate with ring-buffers used as FIFO queues. This device is entirely emulated in QEMU, and can realize DMA transfers by sharing common pages between the guest and the host.
On our v-gpu, we have 2 queues. One dedicated to the hardware cursor, and another for everything else. To send a command in the queue, it goes like this:
- guest: allocate pages on the host
- guest: send a header and pointers to our physical pages (guest POV) in the ring buffer.
- guest: send an interruption
- host: QEMU read our header and pointers. Translate addresses to match local virtual address range.
- host: read the command, execute it
- host: write back to ring buffer
- host: send interruption
- guest: handle interruption, read ring buffer and handle answer
VirGL can be summed up as a simple state-machine, keeping track of resources, and translating command buffers to a sequence of OpenGL calls. It exposes two kinds of commands: let’s say 2D and 3D.
2D commands are mainly focused on resources management. We can allocate memory on the host by creating a 2D resource. Then initialize a DMA transfer by linking this resource’s memory areas to guest’s physical pages. To ease resource management between applications on the guest, VirGL also adds a simple context feature. Resource creation is global, but to use them, you must attach them to the context.
Then, 3D commands. These are close to what we can find in a API like Vulkan. We can setup a viewport, scissor state, create a VBO, and draw it. Shaders are also supported, but we first need to translate them to TGSI; an assembly-like representation. Once on the host, they will be re-translated to GLSL and sent to OpenGL.
You can find a part of the spec on this repository
OpenGL on Windows
Windows graphic stack can be decomposed as follows:
Interresting parts are:
OpenGL ICD (Installable client driver):
This is our OpenGL implementation -> the state machine, which can speak to our kernel driver.
A simple syscall wrapper for us.
First part of the kernel graphic stack. It exposes a 3D and 2D API. Since we are not a licensed developer, let’s try to avoid this. From the documentation, we have a some functions to bypass it: DxgkDdiEscape is one. This functions takes a buffer, a size, and lets it pass trough this subsystem, directly to the underlying driver.
DOD (Display Only Driver)
Our kernel driver. This part will have to communicate to both kernel/ICD and VirtIO-gpu.
OpenGL rely on a state machine we have to implement. Let’s start by drawing on the frame-buffer.
We start a new application, want to split it from the rest. So we start by creating a VirGL context. Then create a 2D resource (800x600 RGBA seams great), and attach it to our VGL-context.
We might want to draw something now. We have two options, either use the 3D command INLINE_WRITE, or DMA. Using INLINE_WRITE means sending all our pixels through a VirtIO queue. So let’s use DMA !
- We start by allocating our memory pages on the guest.
- Then, send physical addresses to VirGL (guest POV)
- VirGL will translate PA addresses to local virtual addresses, and link these pages to our resource.
- Back to the guest, we can write our pixels to the frame-buffer.
- To notify the V-gpu, we use the TRANSFER_TO_HOST_2D command, which tells QEMU to sync resources.
Now, let’s draw some pixels on this frame-buffer. We will need :
- create an OpenGL context
- setup our viewport and scissor settings (ie: screen limits)
- create a VBO
- link the VBO to a vertex/normals/color buffer
- create vertex and frag shaders
- setup a rasterizer
- setup the frame-buffer to use the one we created earlier
- create a constant buffer
- send the draw call
A 3D Command is a set of UINT32. The first one is used as a header, followed by N arguments. A command buffer can contains several commands stacked together in one big UINT32 array.
Earlier, we created resources in VGL-Contexts. Now we will need 3D objects. These are created sending 3D commands, and are not shared between VGL contexts. Once created, we have to bind them to the current opengl-context.
Now, if everything goes well, we should be able to display something like that:
Once more, explaining all the commands would be uninteresting, but there is a spec for that !
If you are still interested, here are couple of links: