Re: [PATCH v6 00/11] vfio/pci: Allow MMIO regions to be exported through dma-buf

Jason Gunthorpe
2025-11-04T00:06:21Z

On Mon, Nov 03, 2025 at 12:07:12PM -0800, Alex Mastro wrote:
> On Sun, Nov 02, 2025 at 10:00:48AM +0200, Leon Romanovsky wrote:
> > Changelog:
> > v6:
> > * Fixed wrong error check from pcim_p2pdma_init().
> > * Documented pcim_p2pdma_provider() function.
> > * Improved commit messages.
> > * Added VFIO DMA-BUF selftest.
> > * Added __counted_by(nr_ranges) annotation to struct vfio_device_feature_dma_buf.
> > * Fixed error unwind when dma_buf_fd() fails.
> > * Document latest changes to p2pmem.
> > * Removed EXPORT_SYMBOL_GPL from pci_p2pdma_map_type.
> > * Moved DMA mapping logic to DMA-BUF.
> > * Removed types patch to avoid dependencies between subsystems.
> > * Moved vfio_pci_dma_buf_move() in err_undo block.
> > * Added nvgrace patch.
>
> Thanks Leon. Attaching a toy program which sanity tests the dma-buf export UAPI
> by feeding the allocated dma-buf into an dma-buf importer (libibverbs + CX-7).

Oh! Here is my toy program to do the same with iommufd as the importer:

#define _GNU_SOURCE
#define __user
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include "include/uapi/linux/vfio.h"
#include "include/uapi/linux/iommufd.h"
#include <string.h>
#include <sys/mman.h>
#include <errno.h>

int main(int argc, const char *argv[])
{
int vfio_dev_fd, iommufd_fd, ret;

// Open the per-device VFIO file (e.g., /dev/vfio/devices/vfio3)
vfio_dev_fd = open("/dev/vfio/devices/vfio0", O_RDWR);
if (vfio_dev_fd < 0) {
perror("Failed to open VFIO per-device file");
return 1;
}

// Open /dev/iommu for iommufd
iommufd_fd = open("/dev/iommu", O_RDWR);
if (iommufd_fd < 0) {
perror("Failed to open /dev/iommu");
close(vfio_dev_fd);
return 1;
}

// Bind device FD to iommufd
struct vfio_device_bind_iommufd bind = {
.argsz = sizeof(bind),
.flags = 0,
.iommufd = iommufd_fd,
};
ret = ioctl(vfio_dev_fd, VFIO_DEVICE_BIND_IOMMUFD, &bind);
if (ret < 0) {
perror("VFIO_DEVICE_BIND_IOMMUFD failed");
close(vfio_dev_fd);
close(iommufd_fd);
return 1;
}

// Allocate an IOAS (I/O address space)
struct iommu_ioas_alloc alloc_data = {
.size = sizeof(alloc_data),
.flags = 0,
};
ret = ioctl(iommufd_fd, IOMMU_IOAS_ALLOC, &alloc_data);
if (ret < 0) {
perror("IOMMU_IOAS_ALLOC failed");
close(vfio_dev_fd);
close(iommufd_fd);
return 1;
}

// Attach the device to the IOAS
struct vfio_device_attach_iommufd_pt attach_data = {
.argsz = sizeof(attach_data),
.flags = 0,
.pt_id = alloc_data.out_ioas_id,
};
ret = ioctl(vfio_dev_fd, VFIO_DEVICE_ATTACH_IOMMUFD_PT, &attach_data);
if (ret < 0) {
perror("VFIO_DEVICE_ATTACH_IOMMUFD_PT failed");
close(vfio_dev_fd);
close(iommufd_fd);
return 1;
}

#if 0
int mapfd = memfd_create("test", MFD_CLOEXEC);
if (mapfd == -1) {
perror("memfd_create failed");
return 1;
}
ftruncate(mapfd, 4096);
#else
struct dmabuf_arg {
struct vfio_device_feature hdr;
struct vfio_device_feature_dma_buf dma_buf;
struct vfio_region_dma_range range;
} dma_buf_feature = {
.hdr = { .argsz = sizeof(dma_buf_feature),
.flags = VFIO_DEVICE_FEATURE_GET |
VFIO_DEVICE_FEATURE_DMA_BUF },
.dma_buf = { .region_index = VFIO_PCI_BAR0_REGION_INDEX,
.open_flags = O_CLOEXEC,
.nr_ranges = 1 },
.range = { .length = 4096 },
};
ret = ioctl(vfio_dev_fd, VFIO_DEVICE_FEATURE, &dma_buf_feature);
if (ret < 0) {
perror("VFIO_DEVICE_FEATURE_GET failed");
return 1;
}
int mapfd = ret;
#endif

struct iommu_ioas_map_file map_file = {
.size = sizeof(map_file),
.flags = IOMMU_IOAS_MAP_WRITEABLE | IOMMU_IOAS_MAP_READABLE,
.ioas_id = alloc_data.out_ioas_id,
.fd = mapfd,
.start = 0,
.length = 4096,
};
ret = ioctl(iommufd_fd, IOMMU_IOAS_MAP_FILE, &map_file);
if (ret < 0) {
perror("IOMMU_IOAS_MAP_FILE failed");
return 1;
}

printf("Successfully attached device to IOAS ID: %u\n",
alloc_data.out_ioas_id);

close(vfio_dev_fd);
close(iommufd_fd);

return 0;
}