In order to implement VFIO_DEVICE_FEATURE_DMA_BUF, we first need
to identify the PCI region 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 in order to invoke
this feature.

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
---
 hw/vfio/region.c              | 49 +++++++++++++++++++++++++++++++++++
 include/hw/vfio/vfio-device.h |  3 +++
 2 files changed, 52 insertions(+)

diff --git a/hw/vfio/region.c b/hw/vfio/region.c
index d04c57db63..b58188147c 100644
--- a/hw/vfio/region.c
+++ b/hw/vfio/region.c
@@ -28,6 +28,7 @@
 #include "qapi/error.h"
 #include "qemu/error-report.h"
 #include "qemu/units.h"
+#include "system/ramblock.h"
 #include "monitor/monitor.h"
 #include "vfio-helpers.h"
 
@@ -401,3 +402,51 @@ void vfio_region_mmaps_set_enabled(VFIORegion *region, bool enabled)
     trace_vfio_region_mmaps_set_enabled(memory_region_name(region->mem),
                                         enabled);
 }
+
+int vfio_device_create_dmabuf(VFIODevice *vdev,
+                              struct iovec *iov, unsigned int iov_cnt)
+{
+    g_autofree struct vfio_device_feature *feature;
+    struct vfio_device_feature_dma_buf *dma_buf;
+    VFIORegion *region;
+    ram_addr_t offset;
+    MemoryRegion *mr;
+    RAMBlock *rb;
+    size_t argsz;
+    int i;
+
+    argsz = sizeof(*feature) + sizeof (*dma_buf) +
+            sizeof(struct vfio_region_dma_range) * iov_cnt;
+    feature = g_malloc0(argsz);
+    dma_buf = (struct vfio_device_feature_dma_buf *)feature->data;
+
+    for (i = 0; i < iov_cnt; i++) {
+        rcu_read_lock();
+        rb = qemu_ram_block_from_host(iov[i].iov_base, false, &offset);
+        rcu_read_unlock();
+
+        if (!rb) {
+            return -1;
+        }
+
+        mr = rb->mr;
+        if (mr->ops != &vfio_region_ops) {
+            mr = mr->container;
+            if (mr->ops != &vfio_region_ops) {
+                return -1;
+            }
+        }
+
+        region = mr->opaque;
+        dma_buf->region_index = region->nr;
+        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;
+    feature->argsz = argsz;
+    feature->flags = VFIO_DEVICE_FEATURE_GET | VFIO_DEVICE_FEATURE_DMA_BUF;
+
+    return ioctl(vdev->fd, VFIO_DEVICE_FEATURE, feature);
+}
diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index 6e4d5ccdac..e127abd5f0 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -277,6 +277,9 @@ bool vfio_device_has_region_cap(VFIODevice *vbasedev, int region, uint16_t cap_t
 
 int vfio_device_get_irq_info(VFIODevice *vbasedev, int index,
                                 struct vfio_irq_info *info);
+
+int vfio_device_create_dmabuf(VFIODevice *vbasedev,
+                              struct iovec *iov, unsigned int iov_cnt);
 #endif
 
 /* Returns 0 on success, or a negative errno. */
-- 
2.50.1