doc/vhost: minor wording tweaks and cleanup

Tweak some wording to be clearer, and add newlines after each section
header for consistency.

Change-Id: I186c7d81b511798838c940c00a18571a08d7fe61
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/368209
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

doc/vhost.md

@@ -2,62 +2,55 @@
 
 # vhost Getting Started Guide {#vhost_getting_started}
 
-The Storage Performance Development Kit vhost application is named "vhost".
+The Storage Performance Development Kit vhost application is named `vhost`.
 This application extends SPDK to present virtio storage controllers to QEMU-based
 VMs and process I/O submitted to devices attached to those controllers.
 
 # Prerequisites {#vhost_prereqs}
 
-The base SPDK build instructions are located README.md in SPDK main directory.
+The base SPDK build instructions are located in README.md in the SPDK root directory.
 This guide assumes familiarity with building SPDK using the default options.
 
 ## Supported Guest Operating Systems
+
 The guest OS must contain virtio drivers. The SPDK vhost target has been tested
 with Ubuntu 16.04, Fedora 25, Windows 2012 R2.
 
 # Building
 
 ## SPDK
-The vhost target is built by default.  To enable/disable building the vhost
-target, either modify the following line in the CONFIG file in the root directory:
-
-~~~
-    CONFIG_VHOST?=y
-~~~
-
-Or specify on the command line:
-
-~~~
-    make CONFIG_VHOST=y
-~~~
 
+The vhost target is built by default.
 Once built, the binary will be at `app/vhost/vhost`.
 
 ## QEMU
 
 Vhost functionality is dependent on QEMU patches to enable virtio-scsi and
-virtio-block in userspace - those patches are currently working their way
+virtio-blk in userspace - those patches are currently working their way
 through the QEMU mailing list, but temporary patches to enable this
 functionality are available in the spdk branch at https://github.com/spdk/qemu.
 
 # Configuration {#vhost_config}
 
 ## SPDK
-A `vhost` specific configuration file is used to configure the SPDK vhost
+
+A vhost-specific configuration file is used to configure the SPDK vhost
 target.  A fully documented example configuration file is located at
 `etc/spdk/vhost.conf.in`.  This file defines the following:
 
 ### Storage Backends
-Storage backends are devices which will be exposed to the guest OS.  In
+
-case of vhost block controller they are exposed as block devices, in case of
+Storage backends are devices which will be exposed to the guest OS.
-vhost scsi as SCSI LUNs on devices attached to the vhost-scsi controller.
+Vhost-blk backends are exposed as block devices in the guest OS, and vhost-scsi backends are
+exposed as as SCSI LUNs on devices attached to the vhost-scsi controller in the guest OS.
 SPDK supports several different types of storage backends, including NVMe,
 Linux AIO, malloc ramdisk and Ceph RBD.  Refer to @ref bdev_getting_started for
 additional information on specifying storage backends in the configuration file.
 
-### Mappings Block Controllers and Storage Backends
+### Mappings Between Block Controllers and Storage Backends
-The vhost target is exposing block controllers to the virtual machines.
+
-The device in the vhost controller is associated with an SPDK block device and
+The vhost target exposes block devices to the virtual machines.
+The device in the vhost controller is associated with an SPDK block device, and the
 configuration file defines those associations.  The block device to Dev mapping
 is specified in the configuration file as:
 
@@ -67,11 +60,11 @@ is specified in the configuration file as:
   Dev BackendX          # "BackendX" is block device name from previous
                         # sections in config file
   #Cpumask 0x1          # Optional parameter defining which core controller uses
-
 ~~~
 
 ### Mappings Between SCSI Controllers and Storage Backends
-The vhost target is exposing SCSI controllers to the virtual machines.
+
+The vhost target exposes SCSI controllers to the virtual machines.
 Each device in the vhost controller is associated with an SPDK block device and
 configuration file defines those associations.  The block device to Dev mappings
 are specified in the configuration file as:
@@ -85,10 +78,10 @@ are specified in the configuration file as:
   ...
   Dev n BackendN
   #Cpumask 0x1          # Optional parameter defining which core controller uses
-
 ~~~
 
 ### Vhost Sockets
+
 Userspace vhost uses UNIX domain sockets for communication between QEMU
 and the vhost target.  Each vhost controller is associated with a UNIX domain
 socket file with filename equal to the Name argument in configuration file.
@@ -96,15 +89,16 @@ Sockets are created at current working directory when starting the SPDK vhost
 target.
 
 ### Core Affinity Configuration
-Vhost target can be restricted to run on certain cores by specifying a ReactorMask.
+
-Default is to allow vhost target work on core 0. For NUMA systems it is essential
+Vhost target can be restricted to run on certain cores by specifying a `ReactorMask`.
+Default is to allow vhost target work on core 0. For NUMA systems, it is essential
 to run vhost with cores on each socket to achieve optimal performance.
 
-To specify which core each controller should use, it can be defined by optional
+Each controller may be assigned a set of cores using the optional
-Cpumask parameter in configuration file.  For NUMA systems the Cpumask should
+`Cpumask` parameter in configuration file.  For NUMA systems, the Cpumask should
-specify cores on the same CPU socket as its associated VM. Application will
+specify cores on the same CPU socket as its associated VM. The `vhost` application will
-pick one core from ReactorMask masked by Cpumask. Cpumask must be subset of
+pick one core from `ReactorMask` masked by `Cpumask`. `Cpumask` must be a subset of
-ReactorMask.
+`ReactorMask`.
 
 ## QEMU
 
@@ -113,7 +107,7 @@ Userspace vhost-scsi adds the following command line option for QEMU:
 -device vhost-user-scsi-pci,id=scsi0,chardev=char0
 ~~~
 
-Userspace vhost-block adds the following command line option for QEMU:
+Userspace vhost-blk adds the following command line option for QEMU:
 ~~~
 -device vhost-user-blk-pci,logical_block_size=4096,size=512M,chardev=char0
 ~~~
@@ -131,6 +125,7 @@ In order to start qemu with vhost you need to specify following options:
 ~~~
 
 # Running Vhost Target
+
 To get started, the following example is usually sufficient:
 ~~~
 app/vhost/vhost -c /path/to/vhost.conf
@@ -143,6 +138,7 @@ app/vhost/vhost -h
 
 
 ## Example
+
 Assume that qemu and spdk are in respectively `qemu` and `spdk` directories.
 ~~~
 ./qemu/build/x86_64-softmmu/qemu-system-x86_64 \
@@ -158,25 +154,27 @@ Assume that qemu and spdk are in respectively `qemu` and `spdk` directories.
         --enable-kvm
 ~~~
 
-# Experimental features {#vhost_experimental}
+# Experimental Features {#vhost_experimental}
 
-## Multi-Queue Block Layer (blk_mq)
+## Multi-Queue Block Layer (blk-mq)
-It is possible to use multiqueue feature in vhost.
+
-To enable it on linux it is required to modify kernel options inside
+It is possible to use the Linux kernel block multi-queue feature with vhost.
+To enable it on Linux, it is required to modify kernel options inside the
 virtual machine.
 
 Instructions below for Ubuntu OS:
 1. `vi /etc/default/grub`
 2. Make sure mq is enabled:
-GRUB_CMDLINE_LINUX="scsi_mod.use_blk_mq=1"
+`GRUB_CMDLINE_LINUX="scsi_mod.use_blk_mq=1"`
 3. `sudo update-grub`
 4. Reboot virtual machine
 
-To achieve better performance make sure to increase number of cores
+To achieve better performance, make sure to increase number of cores
-assigned to vm.
+assigned to the VM.
 
 # Known bugs and limitations {#vhost_bugs}
 
 ## Hot plug is not supported
+
 Hot plug is not supported in vhost yet. Event queue path doesn't handle that
 case yet.