Re: [PATCH v12 10/10] virtio-gpu-dmabuf: Create dmabuf for blobs associated with VFIO devices

[Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>]

On 2026/03/19 14:15, Vivek Kasireddy wrote:
> In addition to memfd, a blob resource can also have its backing
> storage in a VFIO device region. Since, there is no effective way
> to determine where the backing storage is located, we first try to
> create a dmabuf assuming it is in memfd. If that fails, we try to
> create a dmabuf assuming it is in VFIO device region.
> 
> So, we first call virtio_gpu_create_udmabuf() to check if the blob's
> backing storage is located in a memfd or not. If it is not, we invoke
> the vfio_device_create_dmabuf_fd() API which identifies the right
> VFIO device and eventually creates a dmabuf fd.
> 
> Note that, for mmapping the dmabuf, we directly call mmap() if the
> dmabuf fd was created via virtio_gpu_create_udmabuf() since we know
> that the udmabuf driver supports mmap(). However, if the dmabuf was
> created via vfio_device_create_dmabuf_fd(), we use the
> vfio_device_mmap_dmabuf() API to get a mapping for the dmabuf.
> 
> 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@shazbot.org>
> Cc: Cédric Le Goater <clg@redhat.com>
> Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
> ---
>   hw/display/virtio-gpu-dmabuf.c | 23 ++++++++++++++++++++---
>   1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/display/virtio-gpu-dmabuf.c b/hw/display/virtio-gpu-dmabuf.c
> index 89aa487654..f953db0fbe 100644
> --- a/hw/display/virtio-gpu-dmabuf.c
> +++ b/hw/display/virtio-gpu-dmabuf.c
> @@ -147,9 +147,26 @@ void virtio_gpu_init_dmabuf(struct virtio_gpu_simple_resource *res)
>           if (res->dmabuf_fd == VFIO_DMABUF_CREATE_ERR_INVALID_IOV) {
>               error_free_or_abort(&local_err);
>   
> -            qemu_log_mask(LOG_GUEST_ERROR,
> -                          "Cannot create dmabuf: incompatible memory\n");
> -            return;
> +            res->dmabuf_fd = vfio_device_create_dmabuf_fd(res->iov,
> +                                                          res->iov_cnt,
> +                                                          res->blob_size,

The correspondence between (iov, iov_cnt) and blob_size is more of a 
internal concern of virtio-gpu, not of VFIO. This parameter is better 
removed from vfio_device_create_dmabuf_fd() and vfio_device_mmap_dmabuf().

Regards,
Akihiko Odaki

> +                                                          &local_err);
> +            if (res->dmabuf_fd == VFIO_DMABUF_CREATE_ERR_INVALID_IOV) {
> +                error_free_or_abort(&local_err);
> +                qemu_log_mask(LOG_GUEST_ERROR,
> +                              "Cannot create dmabuf: incompatible memory\n");
> +                return;
> +            }
> +
> +            if (res->dmabuf_fd >= 0) {
> +                pdata = vfio_device_mmap_dmabuf(res->iov, res->iov_cnt,
> +                                                res->blob_size, &local_err);
> +                if (!pdata) {
> +                    virtio_gpu_destroy_dmabuf(res);
> +                }
> +            } else {
> +                res->dmabuf_fd = -1;
> +            }
>           } else if (res->dmabuf_fd >= 0) {
>               pdata = virtio_gpu_remap_dmabuf(res, &local_err);
>               if (!pdata) {