docs/system: Add RISC-V documentation

Add RISC-V system emulator documentation for generic information.
`Board-specific documentation` and `RISC-V CPU features` are only
a placeholder and will be added in the future.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210126060007.12904-9-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/target-riscv.rst

@@ -0,0 +1,62 @@
+.. _RISC-V-System-emulator:
+
+RISC-V System emulator
+======================
+
+QEMU can emulate both 32-bit and 64-bit RISC-V CPUs. Use the
+``qemu-system-riscv64`` executable to simulate a 64-bit RISC-V machine,
+``qemu-system-riscv32`` executable to simulate a 32-bit RISC-V machine.
+
+QEMU has generally good support for RISC-V guests. It has support for
+several different machines. The reason we support so many is that
+RISC-V hardware is much more widely varying than x86 hardware. RISC-V
+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.
+
+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 RISC-V system emulation, you must specify which board
+model you want to use with the ``-M`` or ``--machine`` option;
+there is no default.
+
+Because RISC-V 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 user space 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 file system 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 RISC-V boards QEMU supports are currently
+undocumented; you can get a complete list by running
+``qemu-system-riscv64 --machine help``, or
+``qemu-system-riscv32 --machine help``.
+
+RISC-V CPU features
+-------------------

docs/system/targets.rst

@@ -19,6 +19,7 @@ Contents:
    target-m68k
    target-mips
    target-ppc
+   target-riscv
    target-rx
    target-s390x
    target-sparc