verbs: Expose ibv_cmd_export_dmabuf_fd() to be used by drivers

Expose ibv_cmd_export_dmabuf_fd() to be used by drivers to call the
kernel for executing the command.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>

Author: yishaih

libibverbs/CMakeLists.txt

@@ -29,6 +29,7 @@ rdma_library(ibverbs "${CMAKE_CURRENT_BINARY_DIR}/libibverbs.map"
   cmd_cq.c
   cmd_device.c
   cmd_dm.c
+  cmd_dmabuf.c
   cmd_dmah.c
   cmd_fallback.c
   cmd_flow.c

libibverbs/cmd_dmabuf.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0 or Linux-OpenIB
+/*
+ * Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved
+ */
+
+#include <infiniband/cmd_write.h>
+
+int ibv_cmd_export_dmabuf_fd(struct ibv_context *ctx, off_t pg_off)
+{
+	DECLARE_COMMAND_BUFFER(cmd, UVERBS_OBJECT_DMABUF,
+			       UVERBS_METHOD_DMABUF_ALLOC, 2);
+	struct ib_uverbs_attr *handle;
+	int ret;
+
+	handle = fill_attr_out_fd(cmd, UVERBS_ATTR_ALLOC_DMABUF_HANDLE, 0);
+	fill_attr_in_uint64(cmd, UVERBS_ATTR_ALLOC_DMABUF_PGOFF, pg_off);
+
+	ret = execute_ioctl(ctx, cmd);
+	if (ret)
+		/* errno is set to a postive value internally */
+		return -1;
+
+	return read_attr_fd(UVERBS_ATTR_ALLOC_DMABUF_HANDLE, handle);
+}

libibverbs/driver.h

@@ -744,6 +744,7 @@ int ibv_cmd_alloc_dm(struct ibv_context *ctx,
 		     struct verbs_dm *dm,
 		     struct ibv_command_buffer *link);
 int ibv_cmd_free_dm(struct verbs_dm *dm);
+int ibv_cmd_export_dmabuf_fd(struct ibv_context *ctx, off_t pg_off);
 int ibv_cmd_alloc_dmah(struct ibv_context *ctx, struct verbs_dmah *st,
 		       struct ibv_dmah_init_attr *attr);
 int ibv_cmd_free_dmah(struct verbs_dmah *dmah);

libibverbs/libibverbs.map.in

@@ -221,6 +221,7 @@ IBVERBS_PRIVATE_@IBVERBS_PABI_VERSION@ {
 		ibv_cmd_destroy_srq;
 		ibv_cmd_destroy_wq;
 		ibv_cmd_detach_mcast;
+		ibv_cmd_export_dmabuf_fd;
 		ibv_cmd_free_dm;
 		ibv_cmd_free_dmah;
 		ibv_cmd_get_context;