Hi Akihiko,
> Subject: Re: [PATCH v7 10/10] virtio-gpu-dmabuf: Create dmabuf for
> blobs associated with VFIO devices
>
> On 2026/02/23 17:00, 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/meson.build | 1 +
> > hw/display/vfio-dmabuf-stubs.c | 23 +++++++++++++++++++++++
> > hw/display/virtio-gpu-dmabuf.c | 32 ++++++++++++++++++++++++-----
> ---
> > 3 files changed, 48 insertions(+), 8 deletions(-)
> > create mode 100644 hw/display/vfio-dmabuf-stubs.c
> >
> > diff --git a/hw/display/meson.build b/hw/display/meson.build
> > index f5f92b1068..7d2bd839cc 100644
> > --- a/hw/display/meson.build
> > +++ b/hw/display/meson.build
> > @@ -70,6 +70,7 @@ if
> config_all_devices.has_key('CONFIG_VIRTIO_GPU')
> > if_true: [files('virtio-gpu-base.c', 'virtio-gpu.c'),
> > pixman])
> > if host_os == 'linux'
> > virtio_gpu_ss.add(files('virtio-gpu-dmabuf.c'))
> > + virtio_gpu_ss.add(when: 'CONFIG_VFIO', if_false: files('vfio-
> dmabuf-stubs.c'))
> > else
> > virtio_gpu_ss.add(files('virtio-gpu-dmabuf-stubs.c'))
> > endif
> > diff --git a/hw/display/vfio-dmabuf-stubs.c b/hw/display/vfio-dmabuf-
> stubs.c
> > new file mode 100644
> > index 0000000000..12f016f0c5
> > --- /dev/null
> > +++ b/hw/display/vfio-dmabuf-stubs.c
> > @@ -0,0 +1,23 @@
> > +/*
> > + * Copyright (c) 2026 Intel and/or its affiliates.
> > + *
> > + * SPDX-License-Identifier: GPL-2.0-or-later
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +#include "qemu/error-report.h"
> > +#include "qemu/iov.h"
> > +
> > +bool vfio_device_create_dmabuf(struct iovec *iov, unsigned int
> iov_cnt,
> > + int *fd, Error **errp)
> > +{
> > + error_setg(errp, "VFIO dmabuf support is not enabled");
> > + return false;
> > +}
> > +
> > +bool vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int
> iov_cnt,
> > + void **addr, size_t total_size, Error **errp)
> > +{
> > + error_setg(errp, "VFIO mmap dmabuf support is not enabled");
> > + return false;
> > +}
>
> This is VFIO's implementation and shouldn't be here. Follow what
> hw/vfio/iommufd-stubs.c, the previously-mentioned example, does.
Ok, I'll move this file to hw/vfio directory.
>
> > diff --git a/hw/display/virtio-gpu-dmabuf.c b/hw/display/virtio-gpu-
> dmabuf.c
> > index fdcf8e53a2..3f39371de1 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"
> > @@ -82,13 +83,21 @@ virtio_gpu_create_udmabuf(struct
> virtio_gpu_simple_resource *res, int *fd,
> > static bool virtio_gpu_remap_dmabuf(struct
> virtio_gpu_simple_resource *res,
> > Error **errp)
> > {
> > + Error *mmap_err = NULL;
> > 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");
> > + error_setg_errno(&mmap_err, errno, "dmabuf mmap failed: ");
> > res->remapped = NULL;
> > - return false;
> > +
> > + if (!vfio_device_mmap_dmabuf(res->iov, res->iov_cnt, &map,
> > + res->blob_size, errp)) {
> > + error_report_err(mmap_err);
> > + return false;
> > + }
> > + error_free(mmap_err);
> > + mmap_err = NULL;
> > }
> > res->remapped = map;
> > return true;
> > @@ -145,7 +154,7 @@ bool virtio_gpu_have_udmabuf(void)
> >
> > void virtio_gpu_init_dmabuf(struct virtio_gpu_simple_resource *res)
> > {
> > - Error *local_err = NULL;
> > + Error *local_err = NULL, *vfio_err = NULL;
> > VirtIOGPUErrorType err;
> > void *pdata = NULL;
> > int fd;
> > @@ -158,12 +167,19 @@ void virtio_gpu_init_dmabuf(struct
> virtio_gpu_simple_resource *res)
> > err = virtio_gpu_create_udmabuf(res, &fd, &local_err);
> > if (err != VIRTIO_GPU_NO_ERROR) {
> > error_prepend(&local_err, "Cannot create dmabuf: ");
> > - if (err == VIRTIO_GPU_GUEST_ERROR) {
> > - qemu_log_mask(LOG_GUEST_ERROR,
> > - "Cannot create dmabuf: incompatible mem\n");
> > +
> > + if (!vfio_device_create_dmabuf(res->iov, res->iov_cnt, &fd,
> > + &vfio_err)) {
> > + if (err == VIRTIO_GPU_GUEST_ERROR) {
> > + qemu_log_mask(LOG_GUEST_ERROR,
> > + "Cannot create dmabuf: incompatible
> > mem\n");
>
> This logic is faulty. err is not set by vfio_device_create_dmabuf(), so
> it reports a guest error even when the memory is compatible with
> vfio_device_create_dmabuf() and it raised an error for a different
> reason.
Maybe I misunderstood your previous comment about this but I thought
we are going to assume that if both virtio_gpu_create_udmabuf() and
vfio_device_create_dmabuf() fail (assuming Guest error in case of
virtio_gpu_create_udmabuf ()) then it is reasonable to assume that Guest
is at fault for passing in incompatible memory addresses.
However, in order to fix this properly, I think we would have to have
vfio_device_create_dmabuf() return VIRTIO_GPU_GUEST/HOST_ERROR
(or something equivalent) which I was trying to avoid. Do you see any
other proper way to fix this issue without messing with the return type
of vfio_device_create_dmabuf()?
The reason why I am hesitant to make vfio_device_create_dmabuf() return
Guest/Host error is because a future user of this API may not need this info.
Thanks,
Vivek
>
> Regards,
> Akihiko Odaki
>
> > + }
> > + error_report_err(local_err);
> > + error_report_err(vfio_err);
> > + return;
> > }
> > - error_report_err(local_err);
> > - return;
> > + error_free(local_err);
> > + local_err = NULL;
> > }
> >
> > res->dmabuf_fd = fd;
