Hi Gerd, > > Hi, > > > > That sounds sensible to me. Fence the virtio commands, make sure (on > > > the host side) the command completes only when the work is actually done > > > not only submitted. Has recently been added to qemu for RESOURCE_FLUSH > > > (aka frontbuffer rendering) and doing the same for SET_SCANOUT (aka > > > pageflipping), then send vblank events to userspace on command > > > completion certainly makes sense. > > > > Hm how does this all work? At least drm/virtio uses > > drm_atomic_helper_dirtyfb, so both DIRTYFB ioctl and atomic flips all end > > up in the same driver path for everything. Or do you just combine the > > resource_flush with the flip as needed and let the host side figure it all > > out? From a quick read of virtgpu_plane.c that seems to be the case ... > > virtio_gpu_primary_plane_update() will send RESOURCE_FLUSH only for > DIRTYFB and both SET_SCANOUT + RESOURCE_FLUSH for page-flip, and I > think for the page-flip case the host (aka qemu) doesn't get the > "wait until old framebuffer is not in use any more" right yet. [Kasireddy, Vivek] As you know, with the GTK UI backend and this patch series: https://lists.nongnu.org/archive/html/qemu-devel/2021-06/msg06745.html we do create a sync file fd -- after the Blit -- and wait (adding it to Qemu's main event loop) for it to ensure that the Guest scanout FB is longer in use on the Host. This mechanism works in a similarly way for both frontbuffer DIRTYFB case and also the double-buffer case. The out_fence work is only relevant for the future Wayland UI backend though. > > So we'll need a host-side fix for that and a guest-side fix to switch > from a blocking wait on the fence to vblank events. [Kasireddy, Vivek] Do you see any concerns with the blocking wait? And, are you suggesting that we use a vblank timer? Not sure if that would be needed because it would not align with the render/draw signals used with GTK. And, the DRM core does send out an event -- immediately after the blocking wait -- to Guest compositor as no_vblank=true. > > > Also to make this work we don't just need the fence, we need the timestamp > > (in a clock domain the guest can correct for ofc) of the host side kms > > driver flip completion. If you just have the fence then the jitter from > > going through all the layers will most likely make it unusable. > > Well, there are no timestamps in the virtio-gpu protocol ... > > Also I'm not sure they would be that helpful, any timing is *much* less > predictable in a virtual machine, especially in case the host machine is > loaded. [Kasireddy, Vivek] I agree; I think sharing the Host timestamps with the Guest or vice-versa may not be useful. We have not run into any problems without these so far. Thanks, Vivek > > take care, > Gerd
