docs: collect the disparate device emulation docs into one section

While we are at it add a brief preamble that explains some of the
common concepts in QEMU's device emulation which will hopefully lead
to less confusing about our dizzying command line options.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20210720232703.10650-3-alex.bennee@linaro.org>

docs/system/device-emulation.rst

@@ -0,0 +1,89 @@
+.. _device-emulation:
+
+Device Emulation
+----------------
+
+QEMU supports the emulation of a large number of devices from
+peripherals such network cards and USB devices to integrated systems
+on a chip (SoCs). Configuration of these is often a source of
+confusion so it helps to have an understanding of some of the terms
+used to describes devices within QEMU.
+
+Common Terms
+~~~~~~~~~~~~
+
+Device Front End
+================
+
+A device front end is how a device is presented to the guest. The type
+of device presented should match the hardware that the guest operating
+system is expecting to see. All devices can be specified with the
+``--device`` command line option. Running QEMU with the command line
+options ``--device help`` will list all devices it is aware of. Using
+the command line ``--device foo,help`` will list the additional
+configuration options available for that device.
+
+A front end is often paired with a back end, which describes how the
+host's resources are used in the emulation.
+
+Device Buses
+============
+
+Most devices will exist on a BUS of some sort. Depending on the
+machine model you choose (``-M foo``) a number of buses will have been
+automatically created. In most cases the BUS a device is attached to
+can be inferred, for example PCI devices are generally automatically
+allocated to the next free address of first PCI bus found. However in
+complicated configurations you can explicitly specify what bus
+(``bus=ID``) a device is attached to along with its address
+(``addr=N``).
+
+Some devices, for example a PCI SCSI host controller, will add an
+additional buses to the system that other devices can be attached to.
+A hypothetical chain of devices might look like:
+
+  --device foo,bus=pci.0,addr=0,id=foo
+  --device bar,bus=foo.0,addr=1,id=baz
+
+which would be a bar device (with the ID of baz) which is attached to
+the first foo bus (foo.0) at address 1. The foo device which provides
+that bus is itself is attached to the first PCI bus (pci.0).
+
+
+Device Back End
+===============
+
+The back end describes how the data from the emulated device will be
+processed by QEMU. The configuration of the back end is usually
+specific to the class of device being emulated. For example serial
+devices will be backed by a ``--chardev`` which can redirect the data
+to a file or socket or some other system. Storage devices are handled
+by ``--blockdev`` which will specify how blocks are handled, for
+example being stored in a qcow2 file or accessing a raw host disk
+partition. Back ends can sometimes be stacked to implement features
+like snapshots.
+
+While the choice of back end is generally transparent to the guest,
+there are cases where features will not be reported to the guest if
+the back end is unable to support it.
+
+Device Pass Through
+===================
+
+Device pass through is where the device is actually given access to
+the underlying hardware. This can be as simple as exposing a single
+USB device on the host system to the guest or dedicating a video card
+in a PCI slot to the exclusive use of the guest.
+
+
+Emulated Devices
+~~~~~~~~~~~~~~~~
+
+.. toctree::
+   :maxdepth: 1
+
+   devices/ivshmem.rst
+   devices/net.rst
+   devices/nvme.rst
+   devices/usb.rst
+   devices/virtio-pmem.rst

docs/system/index.rst

@@ -11,15 +11,12 @@ or Hypervisor.Framework.
 
    quickstart
    invocation
+   device-emulation
    keys
    mux-chardev
    monitor
    images
-   net
    virtio-net-failover
-   usb
-   nvme
-   ivshmem
    linuxboot
    generic-loader
    guest-loader
@@ -30,7 +27,6 @@ or Hypervisor.Framework.
    gdb
    managed-startup
    cpu-hotplug
-   virtio-pmem
    pr-manager
    targets
    security