Having virtio-gpu import scanout buffers (via prime) from other devices means that we'd be adding a head to headless GPUs assigned to a Guest VM or additional heads to regular GPU devices that are passthrough'd to the Guest. In these cases, the Guest compositor can render into the scanout buffer using a primary GPU and has the secondary GPU (virtio-gpu) import it for display purposes. The main advantage with this is that the imported scanout buffer can either be displayed locally on the Host (e.g, using Qemu + GTK UI) or encoded and streamed to a remote client (e.g, Qemu + Spice UI). Note that since Qemu uses udmabuf driver, there would be no copies made of the scanout buffer as it is displayed. This should be possible even when it might reside in device memory such has VRAM. The specific use-case that can be supported with this series is when running Weston or other guest compositors with "additional-devices" feature (./weston --drm-device=card1 --additional-devices=card0). More info about this feature can be found at: https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/736 In the above scenario, card1 could be a dGPU or an iGPU and card0 would be virtio-gpu in KMS only mode. However, the case where this patch series could be particularly useful is when card1 is a GPU VF that needs to share its scanout buffer (in a zero-copy way) with the GPU PF on the Host. Or, it can also be useful when the scanout buffer needs to be shared between any two GPU devices (assuming one of them is assigned to a Guest VM) as long as they are P2P DMA compatible. As part of the import, the virtio-gpu driver shares the dma addresses and lengths with Qemu which then determines whether the memory region they belong to is owned by a PCI device or whether it is part of the Guest's system ram. If it is the former, it identifies the devid (or bdf) and bar and provides this info (along with offsets and sizes) to the udmabuf driver. In the latter case, instead of the the devid and bar it provides the memfd. The udmabuf driver then creates a dmabuf using this info that Qemu shares with Spice for encode via Gstreamer. Note that the virtio-gpu driver registers a move_notify() callback to track location changes associated with the scanout buffer and sends attach/detach backing cmds to Qemu when appropriate. And, synchronization (that is, ensuring that Guest and Host are not using the scanout buffer at the same time) is ensured by pinning/ unpinning the dmabuf as part of plane update and using a fence in resource_flush cmd. This series is available at: https://gitlab.freedesktop.org/Vivek/drm-tip/-/commits/virtgpu_import_rfc along with additional patches for Qemu and Spice here: https://gitlab.freedesktop.org/Vivek/qemu/-/commits/virtgpu_dmabuf_pcidev https://gitlab.freedesktop.org/Vivek/spice/-/commits/encode_dmabuf_v4 Patchset overview: Patch 1: Implement VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING cmd Patch 2-3: Helpers to initalize, import, free imported object Patch 4-5: Import and use buffers from other devices for scanout Patch 6-7: Have udmabuf driver create dmabuf from PCI bars for P2P DMA This series is tested using the following method: - Run Qemu with the following relevant options: qemu-system-x86_64 -m 4096m .... -device vfio-pci,host=0000:03:00.0 -device virtio-vga,max_outputs=1,blob=true,xres=1920,yres=1080 -spice port=3001,gl=on,disable-ticketing=on,preferred-codec=gstreamer:h264 -object memory-backend-memfd,id=mem1,size=4096M -machine memory-backend=mem1 ... - Run upstream Weston with the following options in the Guest VM: ./weston --drm-device=card1 --additional-devices=card0 where card1 is a DG2 dGPU (passthrough'd and using xe driver in Guest VM), card0 is virtio-gpu and the Host is using a RPL iGPU. Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx> Cc: Dongwon Kim <dongwon.kim@xxxxxxxxx> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> Cc: Christian Koenig <christian.koenig@xxxxxxx> Cc: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> Cc: Rob Clark <robdclark@xxxxxxxxxxxx> Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx> Cc: Oded Gabbay <ogabbay@xxxxxxxxxx> Cc: Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx> Cc: Michael Tretter <m.tretter@xxxxxxxxxxxxxx> Vivek Kasireddy (7): drm/virtio: Implement VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING cmd drm/virtio: Add a helper to map and note the dma addrs and lengths drm/virtio: Add helpers to initialize and free the imported object drm/virtio: Import prime buffers from other devices as guest blobs drm/virtio: Ensure that bo's backing store is valid while updating plane udmabuf/uapi: Add new ioctl to create a dmabuf from PCI bar regions udmabuf: Implement UDMABUF_CREATE_LIST_FOR_PCIDEV ioctl drivers/dma-buf/udmabuf.c | 122 ++++++++++++++++-- drivers/gpu/drm/virtio/virtgpu_drv.h | 8 ++ drivers/gpu/drm/virtio/virtgpu_plane.c | 56 ++++++++- drivers/gpu/drm/virtio/virtgpu_prime.c | 167 ++++++++++++++++++++++++- drivers/gpu/drm/virtio/virtgpu_vq.c | 15 +++ include/uapi/linux/udmabuf.h | 11 +- 6 files changed, 368 insertions(+), 11 deletions(-) -- 2.43.0
