Subject: Re: [PATCH v3 9/9] virtio-gpu-dmabuf: Create dmabuf for blobs
associated with VFIO devices
On 2025/11/22 15:46, Vivek Kasireddy wrote:
In addition to memfd, a blob resource can also have its backing
storage in a VFIO device region. Therefore, we first need to figure
out if the blob is backed by a VFIO device region or a memfd before
we can call the right API to get a dmabuf fd created.
So, we first call virtio_gpu_create_udmabuf() which further calls
ram_block_is_memfd_backed() to check if the blob's backing storage
is located in a memfd or not. If it is not, we call vfio_create_dmabuf()
which identifies the right VFIO device and eventually invokes the
API vfio_device_create_dmabuf_fd() to have a dmabuf fd created.
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
use the VFIO device fd directly to create the CPU mapping.
While at it, remove the unnecessary rcu_read_lock/rcu_read_unlock
from virtio_gpu_create_udmabuf().
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 | 122
++++++++++++++++++++++++++++++---
2 files changed, 119 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 258c48d31b..d121a2c9a7 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"
@@ -40,10 +41,42 @@ static bool ram_block_is_memfd_backed(RAMBlock
*rb)
return false;
}
+static void vfio_create_dmabuf(struct virtio_gpu_simple_resource *res)
+{
+#if defined(VIRTIO_GPU_VFIO_BLOB)
+ VFIODevice *vbasedev;
+ RAMBlock *first_rb;
+ ram_addr_t offset;
+
+ first_rb = qemu_ram_block_from_host(res->iov[0].iov_base, false,
&offset);
+ if (!first_rb) {
+ qemu_log_mask(LOG_GUEST_ERROR,
+ "%s: Could not find ramblock\n", __func__);
+ return;
+ }
+
+ vbasedev = vfio_device_lookup(first_rb->mr);
+ if (!vbasedev) {
+ qemu_log_mask(LOG_GUEST_ERROR,
+ "%s: No VFIO device found to create dmabuf from\n",
+ __func__);
+ return;
+ }
+
+ res->dmabuf_fd = vfio_device_create_dmabuf_fd(vbasedev,
+ res->iov, res->iov_cnt);
+ if (res->dmabuf_fd < 0) {
+ qemu_log_mask(LOG_GUEST_ERROR,
+ "%s: VFIO_DEVICE_FEATURE_DMA_BUF: %s\n",
+ __func__, strerror(errno));
+ }
+#endif
+}
+
static void virtio_gpu_create_udmabuf(struct virtio_gpu_simple_resource
*res)
{
struct udmabuf_create_list *list;
- RAMBlock *rb;
+ RAMBlock *rb, *first_rb;
ram_addr_t offset;
int udmabuf, i;
@@ -52,15 +85,17 @@ static void virtio_gpu_create_udmabuf(struct
virtio_gpu_simple_resource *res)
return;
}
+ first_rb = qemu_ram_block_from_host(res->iov[0].iov_base, false,
&offset);
+ if (!ram_block_is_memfd_backed(first_rb)) {
+ return;
+ }
+
We had an extensive discussion but I still don't understand the benefit
of this change while I see it adds complexity by having another call of
