bdev_virtio: remove internal README.md

# SPDK virtio bdev module

This directory contains an experimental SPDK virtio bdev module.

It currently supports very basic read/write operations for

virtio-scsi drive. Currently it will only work with a single

target with a single LUN.

It 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-scsi controller as a virtual

PCI device.  The virtio-scsi controller might be implemented in the host OS

by SPDK vhost-scsi. Kernel/QEMU vhost-scsi are not supported yet.

* User vhost - Can be used to connect to an SPDK vhost-scsi target running on

the same host.

Note that 1GB hugepages is pretty much required to use this driver in

user-vhost mode.  vhost protocol requires passing a file descriptor for

each region of memory being shared with the vhost target.  Since DPDK opens

every huge page explicitly, it is fairly limited on how many file descriptors

it can pass due to the VHOST_MEMORY_MAX_NREGIONS limit of 8.

Use the following configuration file snippet to enumerate a virtio-scsi PCI

device and present its LUNs as bdevs.

~~~{.sh}

[VirtioPci]

  Enable Yes

~~~

Use the following configuration file snippet to enumerate an SPDK vhost-scsi

controller and present its LUNs as bdevs.  In this case, the SPDK vhost-scsi

target has created an SPDK vhost-scsi controller which is accessible through

the /tmp/vhost.0 domain socket.

~~~{.sh}

[VirtioUser0]

  Path /tmp/vhost.0

~~~

## Todo:

* Add I/O channel support.  Includes requesting correct number of queues

  (based on core count).  Fail device initialization if not enough queues

  can be allocated.

* Add RPCs.

* Break out the "rte_virtio" code into a separate library that is not

  linked directly to the bdev module.  This would allow that part of the

  code to potentially get used and tested outside of the SPDK bdev framework.

* Check for allocation failures in bdev_virtio.c code.

* Add SPDK_TRACELOGs.

* Add virtio-blk support.  This will require some rework in the core

  virtio code (in the rte_virtio subdirectory) to allow for multiple

  device types.

* Bottom out on whether we should have one virtio driver to cover both

  scsi and blk.  If these should be separate, then this driver should be

  renamed to something scsi specific.

* Add reset support.

* Understand and handle queue full conditions.

* Automated test scripts for both PCI and vhost-user scenarios.

* Document Virtio config file section in examples.  Should wait on this until

  enough of the above items are implemented to consider this module as ready

  for more general use.

* Specify the name of the bdev in the config file (and RPC) - currently we

  just hardcode a single bdev name "Virtio0".