In order to mmap a dmabuf, we first need to create a placeholder
mapping that would fit the entire size of the dmabuf. This mapping
would then be replaced with smaller mappings of individual dmabuf
segments.

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              | 65 +++++++++++++++++++++++++++++++++++
 include/hw/vfio/vfio-device.h | 16 +++++++++
 2 files changed, 81 insertions(+)

diff --git a/hw/vfio/device.c b/hw/vfio/device.c
index a7603600f3..ef05ec8ade 100644
--- a/hw/vfio/device.c
+++ b/hw/vfio/device.c
@@ -725,3 +725,68 @@ bool vfio_device_create_dmabuf(struct iovec *iov, unsigned int iov_cnt,
     }
     return true;
 }
+
+bool vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                             void **addr, size_t total_size, Error **errp)
+{
+    struct vfio_region_info *info = NULL;
+    VFIODevice *vbasedev = NULL;
+    ram_addr_t offset, len = 0;
+    RAMBlock *first_rb, *rb;
+    void *map, *submap;
+    int i, index;
+
+    if (!vfio_device_lookup(iov, &vbasedev, errp)) {
+        return false;
+    }
+
+    /*
+     * We first reserve a contiguous chunk of address space for the entire
+     * dmabuf, then replace it with smaller mappings that correspond to the
+     * individual segments of the dmabuf.
+     */
+    map = mmap(NULL, total_size, PROT_NONE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+    if (map == MAP_FAILED) {
+        error_setg_errno(errp, errno, "Could not reserve placeholder mapping");
+        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 mmap dmabuf with different regions\n");
+            goto err;
+        }
+
+        index = vfio_get_region_index_from_mr(rb->mr);
+        if (index < 0) {
+            error_setg(errp, "Cannot find region index for dmabuf segment\n");
+            goto err;
+        }
+
+        if (!vfio_device_get_region_info(vbasedev, index, &info)) {
+            error_setg(errp, "Cannot find region info for dmabuf segment\n");
+            goto err;
+        }
+
+        submap = mmap(map + len, iov[i].iov_len, PROT_READ,
+                      MAP_SHARED | MAP_FIXED, vbasedev->fd,
+                      info->offset + offset);
+        if (submap == MAP_FAILED) {
+            error_setg(errp, "Could not mmap dmabuf segment\n");
+            goto err;
+        }
+
+        len += iov[i].iov_len;
+    }
+    *addr = map;
+    return true;
+err:
+    munmap(map, total_size);
+    error_prepend(errp, "VFIO dmabuf mmap failed: ");
+    return false;
+}
+
diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index 003bac10b5..b8e0801b94 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -332,6 +332,22 @@ int vfio_get_region_index_from_mr(MemoryRegion *mr);
  */
 bool vfio_device_create_dmabuf(struct iovec *iov, unsigned int iov_cnt,
                                int *fd, Error **errp);
+
+/**
+ * Mmap a dmabuf by first translating the addresses in the iovec
+ * array into VFIO region offsets and then creating a placeholder
+ * mapping that would be replaced later with mappings that
+ * correspond to the dmabuf segments.
+ *
+ * @iov: array of iovec entries associated with the dmabuf
+ * @iov_cnt: number of entries in the iovec array
+ * @addr: pointer to store the resulting mmap address
+ * @total_size: total size of the dmabuf
+ *
+ * Returns true on success and false in case of an error.
+ */
+bool vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                             void **addr, size_t total_size, Error **errp);
 #endif
 
 /* Returns 0 on success, or a negative errno. */
-- 
2.50.1