In order to create a dmabuf associated with a buffer that spans
multiple ranges, we first need to identify the VFIO region and
index the buffer (represented by iovec) belongs to and then
translate its addresses to offsets within that region.

The qemu_ram_block_from_host() API gives us both the region and
the offset info we need to populate the dma ranges so that we can
invoke the VFIO_DEVICE_FEATURE_DMA_BUF feature to create the dmabuf.

Cc: Alex Williamson <alex@shazbot.org>
Cc: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
---
 hw/vfio/device.c              | 58 +++++++++++++++++++++++++++++++++++
 include/hw/vfio/vfio-device.h | 14 +++++++++
 2 files changed, 72 insertions(+)

diff --git a/hw/vfio/device.c b/hw/vfio/device.c
index ecb3581fcc..a7603600f3 100644
--- a/hw/vfio/device.c
+++ b/hw/vfio/device.c
@@ -21,6 +21,7 @@
 #include "qemu/osdep.h"
 #include <sys/ioctl.h>
 
+#include "system/ramblock.h"
 #include "hw/vfio/vfio-device.h"
 #include "hw/vfio/pci.h"
 #include "hw/core/iommu.h"
@@ -667,3 +668,60 @@ static bool vfio_device_lookup(struct iovec *iov, VFIODevice **vbasedevp,
     error_setg(errp, "No VFIO device found to create dmabuf from\n");
     return false;
 }
+
+bool vfio_device_create_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                               int *fd, Error **errp)
+{
+    g_autofree struct vfio_device_feature *feature = NULL;
+    struct vfio_device_feature_dma_buf *dma_buf;
+    RAMBlock *rb, *first_rb;
+    VFIODevice *vbasedev;
+    ram_addr_t offset;
+    int i, index;
+    size_t argsz;
+
+    argsz = sizeof(*feature) + sizeof (*dma_buf) +
+            sizeof(struct vfio_region_dma_range) * iov_cnt;
+    feature = g_malloc0(argsz);
+    *feature = (struct vfio_device_feature) {
+        .argsz = argsz,
+        .flags = VFIO_DEVICE_FEATURE_GET | VFIO_DEVICE_FEATURE_DMA_BUF,
+    };
+    dma_buf = (struct vfio_device_feature_dma_buf *)feature->data;
+
+    if (!vfio_device_lookup(iov, &vbasedev, errp)) {
+        return false;
+    }
+
+    for (i = 0; i < iov_cnt; i++) {
+        rb = qemu_ram_block_from_host(iov[i].iov_base, false, &offset);
+        if (i == 0) {
+            first_rb = rb;
+        }
+        if (!rb || rb != first_rb) {
+            error_setg(errp, "Cannot create dmabuf with different regions\n");
+            return false;
+        }
+
+        index = vfio_get_region_index_from_mr(rb->mr);
+        if (index < 0) {
+            error_setg(errp, "Cannot find region index for dmabuf segment\n");
+            return false;
+        }
+
+        dma_buf->region_index = index;
+        dma_buf->dma_ranges[i].offset = offset;
+        dma_buf->dma_ranges[i].length = iov[i].iov_len;
+    }
+
+    dma_buf->nr_ranges = iov_cnt;
+    dma_buf->open_flags = O_RDONLY | O_CLOEXEC;
+
+    *fd = vfio_device_get_feature(vbasedev, feature);
+    if (*fd < 0) {
+        error_setg_errno(errp, -(*fd),
+                         "Could not create dmabuf fd via VFIO device");
+        return false;
+    }
+    return true;
+}
diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index 596c7f5a10..003bac10b5 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -318,6 +318,20 @@ int vfio_device_get_irq_info(VFIODevice *vbasedev, int index,
  * Returns the region index or -1 on error.
  */
 int vfio_get_region_index_from_mr(MemoryRegion *mr);
+
+/**
+ * Create a dmabuf fd by first translating the addresses in the
+ * iovec array into VFIO region offsets and then invoking the
+ * VFIO_DEVICE_FEATURE_DMA_BUF feature.
+ *
+ * @iov: array of iovec entries associated with a buffer
+ * @iov_cnt: number of entries in the iovec array
+ * @fd: pointer to store the resulting dmabuf fd
+ *
+ * Returns true on success and false in case of an error.
+ */
+bool vfio_device_create_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                               int *fd, Error **errp);
 #endif
 
 /* Returns 0 on success, or a negative errno. */
-- 
2.50.1