Adding upstream version 18.2.2.upstream/18.2.2

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

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+# Virtio driver {#virtio}
+
+# Introduction {#virtio_intro}
+
+SPDK Virtio driver is a C library that allows communicating with Virtio devices.
+It allows any SPDK application to become an initiator for (SPDK) vhost targets.
+
+The driver supports two different usage models:
+
+* PCI - This is the standard mode of operation when used in a guest virtual
+  machine, where QEMU has presented the virtio controller as a virtual PCI device.
+* vhost-user - Can be used to connect to a vhost socket directly on the same host.
+
+The driver, just like the SPDK @ref vhost, is using pollers instead of standard
+interrupts to check for an I/O response. If used inside a VM, it bypasses interrupt
+and context switching overhead of QEMU and guest kernel, significantly boosting
+the overall I/O performance.
+
+This Virtio library is currently used to implement two bdev modules:
+@ref bdev_config_virtio_scsi and @ref bdev_config_virtio_blk.
+These modules will export generic SPDK block devices usable by any SPDK application.
+
+# 2MB hugepages {#virtio_2mb}
+
+vhost-user specification puts a limitation on the number of "memory regions" used (8).
+Each region corresponds to one file descriptor, and DPDK - as SPDK's memory allocator -
+uses one file per hugepage by default. So *by default* this makes SPDK Virtio practical
+with only 1GB hugepages. To run an SPDK app using Virtio initiator with 2MB hugepages
+it is required to pass '-g' command-line option . This forces DPDK to create a single
+non-physically-contiguous hugetlbfs file for all its memory.