Adding upstream version 18.2.2.upstream/18.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+# SPDK Porting Guide {#porting}
+
+SPDK is ported to new environments by implementing the *env*
+library interface.  The *env* interface provides APIs for drivers
+to allocate physically contiguous and pinned memory, perform PCI
+operations (config cycles and mapping BARs), virtual to physical
+address translation and managing memory pools.  The *env* API is
+defined in include/spdk/env.h.
+
+SPDK includes a default implementation of the *env* library based
+on the Data Plane Development Kit ([DPDK](http://dpdk.org/)).
+This DPDK implementation can be found in `lib/env_dpdk`.
+
+DPDK is currently supported on Linux and FreeBSD only.
+Users who want to use SPDK on other operating systems, or in
+userspace driver frameworks other than DPDK, will need to implement
+a new version of the *env* library.  The new implementation can be
+integrated into the SPDK build by updating the following line
+in CONFIG:
+
+    CONFIG_ENV?=$(SPDK_ROOT_DIR)/lib/env_dpdk