docs/system/target-arm.rst: Add some introductory text

Now we've moved the various bits of per-board documentation into
their own files, the top level document is a little bare. Add
some introductory information, including a note that many
of the board models we support are currently undocumented.

(Most sections of this new text were originally written by me
for the wiki page https://wiki.qemu.org/Documentation/Platforms/ARM)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200309215818.2021-4-peter.maydell@linaro.org

docs/system/target-arm.rst

@@ -1,9 +1,71 @@
 .. _ARM-System-emulator:
 
-ARM System emulator
+Arm System emulator
 -------------------
 
-Use the executable ``qemu-system-arm`` to simulate a ARM machine.
+QEMU can emulate both 32-bit and 64-bit Arm CPUs. Use the
+``qemu-system-aarch64`` executable to simulate a 64-bit Arm machine.
+You can use either ``qemu-system-arm`` or ``qemu-system-aarch64``
+to simulate a 32-bit Arm machine: in general, command lines that
+work for ``qemu-system-arm`` will behave the same when used with
+``qemu-system-aarch64``.
+
+QEMU has generally good support for Arm guests. It has support for
+nearly fifty different machines. The reason we support so many is that
+Arm hardware is much more widely varying than x86 hardware. Arm CPUs
+are generally built into "system-on-chip" (SoC) designs created by
+many different companies with different devices, and these SoCs are
+then built into machines which can vary still further even if they use
+the same SoC. Even with fifty boards QEMU does not cover more than a
+small fraction of the Arm hardware ecosystem.
+
+The situation for 64-bit Arm is fairly similar, except that we don't
+implement so many different machines.
+
+As well as the more common "A-profile" CPUs (which have MMUs and will
+run Linux) QEMU also supports "M-profile" CPUs such as the Cortex-M0,
+Cortex-M4 and Cortex-M33 (which are microcontrollers used in very
+embedded boards). For most boards the CPU type is fixed (matching what
+the hardware has), so typically you don't need to specify the CPU type
+by hand, except for special cases like the ``virt`` board.
+
+Choosing a board model
+======================
+
+For QEMU's Arm system emulation, you must specify which board
+model you want to use with the ``-M`` or ``--machine`` option;
+there is no default.
+
+Because Arm systems differ so much and in fundamental ways, typically
+operating system or firmware images intended to run on one machine
+will not run at all on any other. This is often surprising for new
+users who are used to the x86 world where every system looks like a
+standard PC. (Once the kernel has booted, most userspace software
+cares much less about the detail of the hardware.)
+
+If you already have a system image or a kernel that works on hardware
+and you want to boot with QEMU, check whether QEMU lists that machine
+in its ``-machine help`` output. If it is listed, then you can probably
+use that board model. If it is not listed, then unfortunately your image
+will almost certainly not boot on QEMU. (You might be able to
+extract the filesystem and use that with a different kernel which
+boots on a system that QEMU does emulate.)
+
+If you don't care about reproducing the idiosyncrasies of a particular
+bit of hardware, such as small amount of RAM, no PCI or other hard
+disk, etc., and just want to run Linux, the best option is to use the
+``virt`` board. This is a platform which doesn't correspond to any
+real hardware and is designed for use in virtual machines. You'll
+need to compile Linux with a suitable configuration for running on
+the ``virt`` board. ``virt`` supports PCI, virtio, recent CPUs and
+large amounts of RAM. It also supports 64-bit CPUs.
+
+Board-specific documentation
+============================
+
+Unfortunately many of the Arm boards QEMU supports are currently
+undocumented; you can get a complete list by running
+``qemu-system-aarch64 --machine help``.
 
 .. toctree::