GPU Passthrough With Libvirt: Disabling the VGA Emulation

448191 · December 21, 2018, 4:18pm

I followed this walkthrough and have a VM with GPU PCI passthrough working, installed the Radeon drivers.

I am already a user of virt-manager and it makes it much easier to manager VMs, generally, so I diverged from the blog post a little in that I used virt-manager and virsh edit.

The problem is that libvirt keeps adding the default VGA emulation (this is documented behavior), which makes the Windows guest prefer that over the Radeon display device. I can set the type of video to none (as suggested by the documentation) but then no output is shown.

Probably this is some really basic ignorance on my part with regard to how video works with libvirt, can somebody enlighten me?

TheCakeIsNaOH · December 21, 2018, 4:20pm

How about disabling the VGA device in windows device manager?

448191 · December 21, 2018, 4:25pm

I tried that, also no video output. So probably the problem is not really the disabling itself, but more “how do I still get video output”. I think it has something to do with the Spice Server Display (similar to VNC) video output which is intended for remote access but I don’t know how to get more direct access to the video output generated by qemu using virt-manager.

TheCakeIsNaOH · December 21, 2018, 4:36pm

Are you trying to use the QEMU window on the Linux host for your display? Or are you using the display out(s) from the back of the passed through GPU?

The QEMU windows does not have 3d acceleration because it always uses the emulated VGA device even if you have a GPU passed through. If you want to have a window on the linux host that has the accelerated GPU display output, then take a look at looking-glass. https://looking-glass.hostfission.com/

448191 · December 21, 2018, 5:11pm

That sound exactly like what I need. I guess I had assumed this would be a standard feature of qemu but I guess that is not too common a use case so many thanks to Geoffrey McRae for developing it.

Online information could be a lot more explicit about needing to either connect a dedicated monitor or installing this framebuffer tool, so thanks to you for pointing me in the right direction.

TheCakeIsNaOH · December 21, 2018, 7:05pm

Well, you came to the right place for looking-glass. See the post below for, some more links to useful threads.

Looking Glass - Guides, Help and Support Linux

Looking Glass What is looking glass? It is a program that uses DXGI (or NvFBC if you're on Quadros or better on Nvidia) to directly capture what is displayed on a video card in a windows virtual machine and then display that on a QEMU/KVM Linux Host in near real-time. Aren't there programs like Remote Desktop, VNC, etc that do this? Those programs provide remote access, yes, but in this case we are copying uncompressed the actual RGBA frames from the render pipeline on the virtual machine for display on the host. It seems similar, but is entirely different. For one we have extremely low latency (probably even less as time goes on since most of our latency seems to be down to the render pipeline in vendors' drivers rather than our own code...). If you are using IOMMU graphics in a virtual machine, and want to use productivity apps on that windows virtual machine at full speed/native performance, you should give this project a look. Known Issues This program is *alpha* quality. It's got bugs. Lots of bugs. It might be more bugs than substrate at this point (not really, but you know). Please read this thread very carefully before posting for help. Many known issues are posted on Geoff's site as well. Note: QEMU currently has a bug in relative-movement mouse pointers that is being worked on. For now, we still recommend using a separate input device (or toggling USB input passthrough between) because of these bugs in QEMU. They’re aware of them, will be fixed soon. Note2: For now, you probably still need something plugged into the passed-through GPU. This seems to depend, a bit, on the GPU but you can use a secondary input on your monitor (often) but not have to actually switch inputs to that display. We have also tested “dummy” EDID DisplayPort plugs as seen in the video, and those work fine, but it is hard to find dummy plugs that have a wide variety of supported display modes. We’ll probably link to some known good ones soon. OMG You Guys Rock! This is the help t…

448191 · December 22, 2018, 6:37pm

Actually, since I’ll need to use a USB toggle for my mouse anyway I might as well just use the KVM I’m not really using. This works great for video.

For mouse and keyboard not so much because I’m passing through the devices, not one of the USB ports. So when the machine is on I can no longer control the host on one of my other 2 monitors.

Still figuring out how to solve this.