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

["Kasireddy, Vivek" <vivek.kasireddy@intel.com>]

Hi Akihiko,

> Subject: Re: [PATCH v12 10/10] virtio-gpu-dmabuf: Create dmabuf for
> blobs associated with VFIO devices
> 
> 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().
I don't disagree. So, should we add the following check in
virtio_gpu_init_dmabuf() or somewhere?
if (iov_size(iov, iov_cnt) != blob_size)

Thanks,
Vivek
> 
> 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) {