On Thu, May 2, 2019 at 6:19 PM Adam Jackson <a...@redhat.com> wrote: > On Thu, 2019-05-02 at 17:05 +0200, Kamil Paral wrote: > > Hello, > > > > I wonder whether it's expected that vsync doesn't work in VMs. I've > > tested Fedora 28/29/30 Workstation and Fedora 30 KDE guests on Fedora > > 30 host, with virtio GPU (3D acceleration on and off) or QXL GPU, and > > in all cases, I'm seeing hundreds or thousands of FPS in glxgears, > > implying that vsync is not working. > > > > Is that an expected state? Is there a simple way to force vsync on > > inside a VM? (using virt-manager + libvirt) > > Short answer: yes it's expected, no there's no way to do that, probably > there should be. > > Long answer: > > The concept of "vsync" in a virtual machine is a bit fuzzy. What > monitor's timings do you think you're syncing to? What do you do when > there's more than one host process watching the framebuffer (think both > gtk and vnc views on the same device)? But, assuming you manage to > answer those questions... > > At least in qemu's implementation neither qxl nor bochs has any > "hardware" concept of a vertical retrace interrupt. And for both of > those you'll end up with llvmpipe for the 3D driver, which doesn't have > any concept of vsync either. The latter we could maybe fix in a few > ways, but I wouldn't expect the former to ever change. > > virtio looks like it has _some_ internal notion of interrupts, but if > that's supposed to be implementing vsync it's not obvious to me how > it's accomplishing that. vmwgfx I suspect does have synthetic vsync > events, but I think you'd need to be using an actual vmware vm to make > that happen and not qemu. >
Thanks, Adam, for a detailed answer.
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