docs: add an introduction to the system docs

Drop the frankly misleading quickstart section for a more rounded
introduction section. This new section gives an overview of the
accelerators as well as a high level introduction to some of the key
features of the emulator. We also expand on a general form for a QEMU
command line with a hopefully not too scary worked example of what
this looks like.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20230124180127.1881110-23-alex.bennee@linaro.org>

6 files changed, 227 insertions, 22 deletions

diff --git a/docs/interop/live-block-operations.rst b/docs/interop/live-block-operations.rst
index 135784a..691429c 100644
--- a/docs/interop/live-block-operations.rst
+++ b/docs/interop/live-block-operations.rst
@@ -4,6 +4,8 @@
     This work is licensed under the terms of the GNU GPL, version 2 or
     later.  See the COPYING file in the top-level directory.
 
+.. _Live Block Operations:
+
 ============================
 Live Block Device Operations
 ============================
diff --git a/docs/interop/qemu-qmp-ref.rst b/docs/interop/qemu-qmp-ref.rst
index 357effd..f94614a 100644
--- a/docs/interop/qemu-qmp-ref.rst
+++ b/docs/interop/qemu-qmp-ref.rst
@@ -1,3 +1,5 @@
+.. _QMP Ref:
+
 QEMU QMP Reference Manual
 =========================
 
diff --git a/docs/system/index.rst b/docs/system/index.rst
index 282b6ff..3605bbe 100644
--- a/docs/system/index.rst
+++ b/docs/system/index.rst
@@ -12,7 +12,7 @@ or Hypervisor.Framework.
 .. toctree::
    :maxdepth: 3
 
-   quickstart
+   introduction
    invocation
    device-emulation
    keys
diff --git a/docs/system/introduction.rst b/docs/system/introduction.rst
new file mode 100644
index 0000000..c8a9fe6
--- /dev/null
+++ b/docs/system/introduction.rst
@@ -0,0 +1,220 @@
+Introduction
+============
+
+Virtualisation Accelerators
+---------------------------
+
+QEMU's system emulation provides a virtual model of a machine (CPU,
+memory and emulated devices) to run a guest OS. It supports a number
+of hypervisors (known as accelerators) as well as a JIT known as the
+Tiny Code Generator (TCG) capable of emulating many CPUs.
+
+.. list-table:: Supported Accelerators
+  :header-rows: 1
+
+  * - Accelerator
+    - Host OS
+    - Host Architectures
+  * - KVM
+    - Linux
+    - Arm (64 bit only), MIPS, PPC, RISC-V, s390x, x86
+  * - Xen
+    - Linux (as dom0)
+    - Arm, x86
+  * - Intel HAXM (hax)
+    - Linux, Windows
+    - x86
+  * - Hypervisor Framework (hvf)
+    - MacOS
+    - x86 (64 bit only), Arm (64 bit only)
+  * - Windows Hypervisor Platform (wphx)
+    - Windows
+    - x86
+  * - NetBSD Virtual Machine Monitor (nvmm)
+    - NetBSD
+    - x86
+  * - Tiny Code Generator (tcg)
+    - Linux, other POSIX, Windows, MacOS
+    - Arm, x86, Loongarch64, MIPS, PPC, s390x, Sparc64
+
+Feature Overview
+----------------
+
+System emulation provides a wide range of device models to emulate
+various hardware components you may want to add to your machine. This
+includes a wide number of VirtIO devices which are specifically tuned
+for efficient operation under virtualisation. Some of the device
+emulation can be offloaded from the main QEMU process using either
+vhost-user (for VirtIO) or :ref:`Multi-process QEMU`. If the platform
+supports it QEMU also supports directly passing devices through to
+guest VMs to eliminate the device emulation overhead. See
+:ref:`device-emulation` for more details.
+
+There is a full :ref:`featured block layer<Live Block Operations>`
+which allows for construction of complex storage topology which can be
+stacked across multiple layers supporting redirection, networking,
+snapshots and migration support.
+
+The flexible ``chardev`` system allows for handling IO from character
+like devices using stdio, files, unix sockets and TCP networking.
+
+QEMU provides a number of management interfaces including a line based
+:ref:`Human Monitor Protocol (HMP)<QEMU monitor>` that allows you to
+dynamically add and remove devices as well as introspect the system
+state. The :ref:`QEMU Monitor Protocol<QMP Ref>` (QMP) is a well
+defined, versioned, machine usable API that presents a rich interface
+to other tools to create, control and manage Virtual Machines. This is
+the interface used by higher level tools interfaces such as `Virt
+Manager <https://virt-manager.org/>`_ using the `libvirt framework
+<https://libvirt.org>`_.
+
+For the common accelerators QEMU, supported debugging with its
+:ref:`gdbstub<GDB usage>` which allows users to connect GDB and debug
+system software images.
+
+Running
+-------
+
+QEMU provides a rich and complex API which can be overwhelming to
+understand. While some architectures can boot something with just a
+disk image, those examples elide a lot of details with defaults that
+may not be optimal for modern systems.
+
+For a non-x86 system where we emulate a broad range of machine types,
+the command lines are generally more explicit in defining the machine
+and boot behaviour. You will find often find example command lines in
+the :ref:`system-targets-ref` section of the manual.
+
+While the project doesn't want to discourage users from using the
+command line to launch VMs, we do want to highlight that there are a
+number of projects dedicated to providing a more user friendly
+experience. Those built around the ``libvirt`` framework can make use
+of feature probing to build modern VM images tailored to run on the
+hardware you have.
+
+That said, the general form of a QEMU command line can be expressed
+as:
+
+.. parsed-literal::
+
+  $ |qemu_system| [machine opts] \\
+                  [cpu opts] \\
+                  [accelerator opts] \\
+                  [device opts] \\
+                  [backend opts] \\
+                  [interface opts] \\
+                  [boot opts]
+
+Most options will generate some help information. So for example:
+
+.. parsed-literal::
+
+   $ |qemu_system| -M help
+
+will list the machine types supported by that QEMU binary. ``help``
+can also be passed as an argument to another option. For example:
+
+.. parsed-literal::
+
+  $ |qemu_system| -device scsi-hd,help
+
+will list the arguments and their default values of additional options
+that can control the behaviour of the ``scsi-hd`` device.
+
+.. list-table:: Options Overview
+  :header-rows: 1
+  :widths: 10, 90
+
+  * - Options
+    -
+  * - Machine
+    - Define the machine type, amount of memory etc
+  * - CPU
+    - Type and number/topology of vCPUs. Most accelerators offer
+      a ``host`` cpu option which simply passes through your host CPU
+      configuration without filtering out any features.
+  * - Accelerator
+    - This will depend on the hypervisor you run. Note that the
+      default is TCG, which is purely emulated, so you must specify an
+      accelerator type to take advantage of hardware virtualization.
+  * - Devices
+    - Additional devices that are not defined by default with the
+      machine type.
+  * - Backends
+    - Backends are how QEMU deals with the guest's data, for example
+      how a block device is stored, how network devices see the
+      network or how a serial device is directed to the outside world.
+  * - Interfaces
+    - How the system is displayed, how it is managed and controlled or
+      debugged.
+  * - Boot
+    - How the system boots, via firmware or direct kernel boot.
+
+In the following example we first define a ``virt`` machine which is a
+general purpose platform for running Aarch64 guests. We enable
+virtualisation so we can use KVM inside the emulated guest. As the
+``virt`` machine comes with some built in pflash devices we give them
+names so we can override the defaults later.
+
+.. code::
+
+ $ qemu-system-aarch64 \
+    -machine type=virt,virtualization=on,pflash0=rom,pflash1=efivars \
+    -m 4096 \
+
+We then define the 4 vCPUs using the ``max`` option which gives us all
+the Arm features QEMU is capable of emulating. We enable a more
+emulation friendly implementation of Arm's pointer authentication
+algorithm. We explicitly specify TCG acceleration even though QEMU
+would default to it anyway.
+
+.. code::
+
+ -cpu max,pauth-impdef=on \
+ -smp 4 \
+ -accel tcg \
+
+As the ``virt`` platform doesn't have any default network or storage
+devices we need to define them. We give them ids so we can link them
+with the backend later on.
+
+.. code::
+
+ -device virtio-net-pci,netdev=unet \
+ -device virtio-scsi-pci \
+ -device scsi-hd,drive=hd \
+
+We connect the user-mode networking to our network device. As
+user-mode networking isn't directly accessible from the outside world
+we forward localhost port 2222 to the ssh port on the guest.
+
+.. code::
+
+ -netdev user,id=unet,hostfwd=tcp::2222-:22 \
+
+We connect the guest visible block device to an LVM partition we have
+set aside for our guest.
+
+.. code::
+
+ -blockdev driver=raw,node-name=hd,file.driver=host_device,file.filename=/dev/lvm-disk/debian-bullseye-arm64 \
+
+We then tell QEMU to multiplex the :ref:`QEMU monitor` with the serial
+port output (we can switch between the two using :ref:`keys in the
+character backend multiplexer`). As there is no default graphical
+device we disable the display as we can work entirely in the terminal.
+
+.. code::
+
+ -serial mon:stdio \
+ -display none \
+
+Finally we override the default firmware to ensure we have some
+storage for EFI to persist its configuration. That firmware is
+responsible for finding the disk, booting grub and eventually running
+our system.
+
+.. code::
+
+ -blockdev node-name=rom,driver=file,filename=(pwd)/pc-bios/edk2-aarch64-code.fd,read-only=true \
+ -blockdev node-name=efivars,driver=file,filename=$HOME/images/qemu-arm64-efivars
diff --git a/docs/system/multi-process.rst b/docs/system/multi-process.rst
index 210531e..16f0352 100644
--- a/docs/system/multi-process.rst
+++ b/docs/system/multi-process.rst
@@ -1,3 +1,5 @@
+.. _Multi-process QEMU:
+
 Multi-process QEMU
 ==================
 
diff --git a/docs/system/quickstart.rst b/docs/system/quickstart.rst
deleted file mode 100644
index 681678c..0000000
--- a/docs/system/quickstart.rst
+++ /dev/null
@@ -1,21 +0,0 @@
-.. _pcsys_005fquickstart:
-
-Quick Start
------------
-
-Download and uncompress a PC hard disk image with Linux installed (e.g.
-``linux.img``) and type:
-
-.. parsed-literal::
-
-   |qemu_system| linux.img
-
-Linux should boot and give you a prompt.
-
-Users should be aware the above example elides a lot of the complexity
-of setting up a VM with x86_64 specific defaults and assumes the
-first non switch argument is a PC compatible disk image with a boot
-sector. For a non-x86 system where we emulate a broad range of machine
-types, the command lines are generally more explicit in defining the
-machine and boot behaviour. You will find more example command lines
-in the :ref:`system-targets-ref` section of the manual.