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              | 44 +++++++++++++++++++++++++++++++++++
 include/hw/vfio/vfio-device.h | 14 +++++++++++
 2 files changed, 58 insertions(+)

diff --git a/hw/vfio/device.c b/hw/vfio/device.c
index afeff0f034..90d7758d0d 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"
@@ -656,3 +657,46 @@ VFIODevice *vfio_device_lookup(MemoryRegion *mr)
     }
     return NULL;
 }
+
+int vfio_device_create_dmabuf_fd(VFIODevice *vbasedev,
+                                 struct iovec *iov, unsigned int iov_cnt)
+{
+    g_autofree struct vfio_device_feature *feature = NULL;
+    struct vfio_device_feature_dma_buf *dma_buf;
+    RAMBlock *rb, *first_rb = NULL;
+    ram_addr_t offset;
+    size_t argsz;
+    int i, index;
+
+    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;
+
+    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) {
+            return -1;
+        }
+
+        index = vfio_get_region_index_from_mr(rb->mr);
+        if (index < 0) {
+            return -1;
+        }
+
+        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;
+    return vfio_device_get_feature(vbasedev, feature);
+}
diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index ccf2fd887c..2f7e50f7ce 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -327,6 +327,20 @@ int vfio_get_region_index_from_mr(MemoryRegion *mr);
  * Returns the VFIO device if found or NULL.
  */
 VFIODevice *vfio_device_lookup(MemoryRegion *mr);
+
+/**
+ * Create and return 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.
+ *
+ * @vbasedev: #VFIODevice to use
+ * @iov: array of iovec entries associated with a buffer
+ * @iov_cnt: number of entries in the iovec array
+ *
+ * Returns the newly created dmabuf fd or -1 on error.
+ */
+int vfio_device_create_dmabuf_fd(VFIODevice *vbasedev,
+                                 struct iovec *iov, unsigned int iov_cnt);
 #endif
 
 /* Returns 0 on success, or a negative errno. */
-- 
2.50.1