On 10/9/25 06:23, Kasireddy, Vivek wrote: > Hi Thomas, > >> Subject: [PATCH] drm/virtgpu: Use vblank timer >> >> Use a vblank timer to simulate the vblank interrupt. The DRM vblank >> helpers provide an implementation on top of Linux' hrtimer. Virtgpu >> enables and disables the timer as part of the CRTC. The atomic_flush >> callback sets up the event. Like vblank interrupts, the vblank timer >> fires at the rate of the display refresh. >> >> Most userspace limits its page flip rate according to the DRM vblank >> event. Virtgpu's virtual hardware does not provide vblank interrupts, so >> DRM sends each event ASAP. With the fast access times of virtual display >> memory, the event rate is much higher than the display mode's refresh >> rate; creating the next page flip almost immediately. This leads to >> excessive CPU overhead from even small display updates, such as moving >> the mouse pointer. >> >> This problem affects virtgpu and all other virtual displays. See [1] for >> a discussion in the context of hypervdrm. > When running Virtio-gpu with some of the UIs (gtk, spice, etc) in Qemu, the > Guest display updates are synchronized with a timer (or repaint callback in > the case of Gtk) the UI layer provides particularly when blob=true. In other > words, the Guest's atomic commit does not complete until the Host signals > (via a fence) that the update (or flush) is done. > > This is because when blob=true, the Guest's FB is accessed by the Host without > making any copies. So, to avoid tearing, the Guest needs to be prevented from > accessing its FB until the Host is done. Therefore, I think for virtio-gpu, > the virtual > vblank timer can only be helpful when blob=false as this is the only case > where > Guest's display updates are unbounded (and Host makes a copy of the Guest's > FB). > > I am not sure how this would affect virgl use-cases but Dmitry can help > explain if > the vblank timer would be useful there or not.
The vblank timer should only limit framerate of virtio-gpu, I don't expect it will cause new problems. Do you see tearing using this patch? Vblank timer makes a big performance improvement for virtio-gpu KMS, especially for a native context on QEMU. No tearing spotted with it. -- Best regards, Dmitry
