Re: [PATCH v1 0/7] vfio: Implement VFIO_DEVICE_FEATURE_DMA_BUF and use it in virtio-gpu

["Cédric Le Goater" <clg@redhat.com>]

Vivek,

On 10/4/25 01:35, Vivek Kasireddy wrote:
> The virtio-gpu driver running in the Guest VM can create Guest blob
> resources (by importing dmabufs) that are backed by System RAM. This
> is made possible by making use of memfd memory backend and udmabuf
> driver on the Host side. However, in order to create Guest blobs
> that are backed by vfio-pci device regions (which happens when
> virtio-gpu imports dmabufs from devices that have local memory such
> as dGPU VFs), we have to implement VFIO_DEVICE_FEATURE_DMA_BUF and
> leverage it in virtio-gpu.
> 
> So, while creating the blobs we use memory_region_is_ram_device() to
> figure out if the blob is backed by memfd or a vfio-pci device. If
> it is determined that the blob is backed by vfio-pci device region,
> instead of calling into udmabuf driver to create dmabuf we would
> now call into vfio-pci driver to have a dmabuf created on the Host.
> 
> RFC -> v1:
> - Create the CPU mapping using vfio device fd if the dmabuf exporter
>    (vfio-pci) does not provide mmap() support (Akihiko)
> - Log a warning with LOG_GUEST_ERROR instead of warn_report() when
>    dmabuf cannot be created using Guest provided addresses (Akihiko)
> - Use address_space_translate() instead of gpa2hva() to obtain the
>    Host addresses (Akihiko)
> - Rearrange the patches and improve the commit messages (Akihiko)
> - Fix compilation error when VFIO is not enabled (Alex)
> - Add a new helper to obtain VFIO region index from memory region
> - Move vfio_device_create_dmabuf() to hw/vfio/device.c
> 
> Tested with an SRIOV enabled Intel dGPU by running Gnome Wayland

Could you please be more precise on the HW ?

> (in the VM) and Qemu with the following (relevant) parameters:
> -device vfio-pci,host=0000:03:00.1
> -device virtio-vga,max_outputs=1,xres=1920,yres=1080,blob=true
> -display gtk,gl=on
> 
> Associated vfio-pci kernel driver series:
> https://lore.kernel.org/dri-devel/cover.1754311439.git.leon@kernel.org/
> Associated virtio-gpu kernel driver series (merged):
> https://lore.kernel.org/dri-devel/20241126031643.3490496-1-vivek.kasireddy@intel.com/
> 
> ---
> Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
> Cc: Alex Bennée <alex.bennee@linaro.org>
> Cc: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: Cédric Le Goater <clg@redhat.com>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: Leon Romanovsky <leonro@nvidia.com>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Dongwon Kim <dongwon.kim@intel.com>
> 
> Vivek Kasireddy (7):
>    virtio-gpu: Recreate the resource's dmabuf if new backing is attached
>    virtio-gpu: Don't rely on res->blob to identify blob resources
>    virtio-gpu: Find hva for Guest's DMA addr associated with a ram device
>    vfio/region: Add a helper to get region index from memory region
>    linux-headers: Update vfio.h to include VFIO_DEVICE_FEATURE_DMA_BUF
>    vfio/device: Add support for VFIO_DEVICE_FEATURE_DMA_BUF
>    virtio-gpu-udmabuf: Create dmabuf for blobs associated with VFIO
>      devices
> 
>   hw/display/Kconfig              |   5 ++
>   hw/display/virtio-gpu-udmabuf.c | 143 ++++++++++++++++++++++++++++++--
>   hw/display/virtio-gpu.c         |  53 +++++++++---
>   hw/vfio/device.c                |  43 ++++++++++
>   hw/vfio/region.c                |  14 ++++
>   include/hw/vfio/vfio-device.h   |   5 ++
>   linux-headers/linux/vfio.h      |  25 ++++++
>   7 files changed, 270 insertions(+), 18 deletions(-)
> 

Please cc: Alex and I on all patches.

Thanks,

C.