Hi Akihiko,
> Subject: Re: [PATCH v6 11/11] virtio-gpu-dmabuf: Create dmabuf for
> blobs associated with VFIO devices
>
> On 2026/02/10 15:59, 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() API which identifies the right VFIO
> > device and eventually creates a dmabuf fd.
> >
> > Note that in virtio_gpu_remap_dmabuf(), we first try to test if
> > the VFIO dmabuf exporter supports mmap or not. If it doesn't, we
> > fall back to the vfio_device_mmap_dmabuf() API to get a mapping
> > created for the dmabuf.
> >
> > Cc: Marc-André Lureau <[email protected]>
> > Cc: Alex Bennée <[email protected]>
> > Cc: Akihiko Odaki <[email protected]>
> > Cc: Dmitry Osipenko <[email protected]>
> > Cc: Alex Williamson <[email protected]>
> > Cc: Cédric Le Goater <[email protected]>
> > Signed-off-by: Vivek Kasireddy <[email protected]>
> > ---
> > hw/display/Kconfig | 5 ++++
> > hw/display/virtio-gpu-dmabuf.c | 50
> ++++++++++++++++++++++++++++------
> > 2 files changed, 47 insertions(+), 8 deletions(-)
> >
> > diff --git a/hw/display/Kconfig b/hw/display/Kconfig
> > index 1e95ab28ef..0d090f25f5 100644
> > --- a/hw/display/Kconfig
> > +++ b/hw/display/Kconfig
> > @@ -106,6 +106,11 @@ config VIRTIO_VGA
> > depends on VIRTIO_PCI
> > select VGA
> >
> > +config VIRTIO_GPU_VFIO_BLOB
> > + bool
> > + default y
> > + depends on VFIO
> > +
> > config VHOST_USER_GPU
> > bool
> > default y
> > diff --git a/hw/display/virtio-gpu-dmabuf.c b/hw/display/virtio-gpu-
> dmabuf.c
> > index d9b2ecaf31..54d010b995 100644
> > --- a/hw/display/virtio-gpu-dmabuf.c
> > +++ b/hw/display/virtio-gpu-dmabuf.c
> > @@ -18,6 +18,7 @@
> > #include "ui/console.h"
> > #include "hw/virtio/virtio-gpu.h"
> > #include "hw/virtio/virtio-gpu-pixman.h"
> > +#include "hw/vfio/vfio-device.h"
> > #include "trace.h"
> > #include "system/ramblock.h"
> > #include "system/hostmem.h"
> > @@ -27,6 +28,27 @@
> > #include "standard-headers/linux/udmabuf.h"
> > #include "standard-headers/drm/drm_fourcc.h"
> >
> > +static bool virtio_gpu_create_vfio_dmabuf(struct
> virtio_gpu_simple_resource *r,
> > + Error **errp)
> > +{
> > + bool ret = false;
> > +#if defined(CONFIG_VIRTIO_GPU_VFIO_BLOB)
> > + Error *local_err = NULL;
> > + int fd;
> > +
> > + if (!vfio_device_create_dmabuf(r->iov, r->iov_cnt, &fd, &local_err))
> {
> > + error_report_err(*errp);
> > + *errp = NULL;
> > + error_propagate(errp, local_err);
> > + return false;
> > + }
> > +
> > + ret = true;
> > + r->dmabuf_fd = fd;
> > +#endif
> > + return ret;
> > +}
> > +
> > static bool virtio_gpu_create_udmabuf(struct
> virtio_gpu_simple_resource *res,
> > Error **errp)
> > {
> > @@ -73,16 +95,28 @@ static bool virtio_gpu_create_udmabuf(struct
> virtio_gpu_simple_resource *res,
> > static bool virtio_gpu_remap_dmabuf(struct
> virtio_gpu_simple_resource *res,
> > Error **errp)
> > {
> > + Error *local_err = NULL;
> > + bool ret = true;
> > void *map;
> >
> > map = mmap(NULL, res->blob_size, PROT_READ, MAP_SHARED,
> res->dmabuf_fd, 0);
> > if (map == MAP_FAILED) {
> > error_setg_errno(errp, errno, "dmabuf mmap failed");
> > - res->remapped = NULL;
> > - return false;
> > + map = NULL;
> > + ret = false;
> > +#if defined(CONFIG_VIRTIO_GPU_VFIO_BLOB)
> > + if (vfio_device_mmap_dmabuf(res->iov, res->iov_cnt, &map,
> > + res->blob_size, &local_err)) {
> > + ret = true;
> > + } else {
> > + error_report_err(*errp);
> > + *errp = NULL;
> > + }
> > +#endif
> > + error_propagate(errp, local_err);
> > }
> > res->remapped = map;
> > - return true;
> > + return ret;
> > }
> >
> > static void virtio_gpu_destroy_dmabuf(struct
> virtio_gpu_simple_resource *res)
> > @@ -145,8 +179,10 @@ void virtio_gpu_init_dmabuf(struct
> virtio_gpu_simple_resource *res)
> > pdata = res->iov[0].iov_base;
> > } else {
> > if (!virtio_gpu_create_udmabuf(res, &local_err)) {
>
> virtio_gpu_create_udmabuf() will log "Could not find valid ramblock"
> even for VFIO, which shouldn't happen.
AFAICS, it won't log a failure here because rb->fd is valid in the VFIO case
especially after 8cfaf22668 ("Create dmabuf for PCI BAR per region").
The failure would occur in UDMABUF_CREATE_LIST IOCTL most likely
because rb->fd is not memfd in this case.
>
> This logic should look like as follows:
>
> virtio_gpu_create_udmabuf(res);
> if (it turned out the memory is incompatible with udmabuf) {
> virtio_gpu_create_vfio_dmabuf(res);
> if (it turned out the memory is not backed by VFIO) {
> qemu_log_mask(LOG_GUEST_ERROR, "incompatible\n");
> return;
I think this is mostly how I have it right now except that I am using
error_report_err() instead of qemu_log_mask(). Do you want me
to only use qemu_log_mask() when we cannot create a dmabuf via
udmabuf and vfio?
As you can see, I am using a mix of error_report_err() and qemu_log_mask()
after Cedric suggested to improve error handling by using 'Error *'.
On a slightly different note, do you (or Cedric/anyone) know how to
properly include CONFIG_DEVICES header to fix the following error:
../hw/display/virtio-gpu-dmabuf.c: In function 'virtio_gpu_create_vfio_dmabuf':
../hw/display/virtio-gpu-dmabuf.c:35:13: error: attempt to use poisoned
'CONFIG_VIRTIO_GPU_VFIO_BLOB'
35 | #if defined(CONFIG_VIRTIO_GPU_VFIO_BLOB)
Looks like I am missing some key detail as I have tried different things but
have not been able to fix this compilation error.
Thanks,
Vivek
> }
> }
>
> Regards,
> Akihiko Odaki
>
> > - if (local_err) {
> > - error_report_err(local_err);
> > + if (!virtio_gpu_create_vfio_dmabuf(res, &local_err)) {
> > + if (local_err) {
> > + error_report_err(local_err);
> > + }
> > }
> > return;
> > }
> > @@ -163,9 +199,7 @@ void virtio_gpu_init_dmabuf(struct
> virtio_gpu_simple_resource *res)
> >
> > void virtio_gpu_fini_dmabuf(struct virtio_gpu_simple_resource *res)
> > {
> > - if (res->remapped) {
> > - virtio_gpu_destroy_dmabuf(res);
> > - }
> > + virtio_gpu_destroy_dmabuf(res);
> > }
> >
> > static void virtio_gpu_free_dmabuf(VirtIOGPU *g, VGPUDMABuf
> *dmabuf)
