On 2026/02/25 15:03, Kasireddy, Vivek wrote:
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.
It's not what I suggested. What I suggested is 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;
> }
> }
https://lore.kernel.org/qemu-devel/[email protected]/
It is not correct to assume vfio_device_create_dmabuf() is failed due to
a guest error when virtio_gpu_create_udmabuf(). For example, when mmap()
fails in vfio_device_mmap_dmabuf(), it is a host error, and reporting a
guest error in the case is a bug.
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.
A future user may not need it, but we need it to make this code bug-free.
Regards,
Akihiko Odaki
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;
