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

[Jason Gunthorpe]

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;
}