virtio-pmem: Add virtio pmem driver

This patch adds virtio-pmem driver for KVM guest.

Guest reads the persistent memory range information from
Qemu over VIRTIO and registers it on nvdimm_bus. It also
creates a nd_region object with the persistent memory
range information so that existing 'nvdimm/pmem' driver
can reserve this into system memory map. This way
'virtio-pmem' driver uses existing functionality of pmem
driver to register persistent memory compatible for DAX
capable filesystems.

This also provides function to perform guest flush over
VIRTIO from 'pmem' driver when userspace performs flush
on DAX memory range.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jakub Staron <jstaron@google.com>
Tested-by: Jakub Staron <jstaron@google.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/virtio/Kconfig

@@ -43,6 +43,17 @@ config VIRTIO_PCI_LEGACY
 
 	  If unsure, say Y.
 
+config VIRTIO_PMEM
+	tristate "Support for virtio pmem driver"
+	depends on VIRTIO
+	depends on LIBNVDIMM
+	help
+	  This driver provides access to virtio-pmem devices, storage devices
+	  that are mapped into the physical address space - similar to NVDIMMs
+	   - with a virtio-based flushing interface.
+
+	  If unsure, say Y.
+
 config VIRTIO_BALLOON
 	tristate "Virtio balloon driver"
 	depends on VIRTIO