Re: [PATCH 21/22] docs/system: Add RISC-V documentation

[Alistair Francis <alistair23@gmail.com>]

On Thu, Dec 31, 2020 at 3:53 AM Bin Meng <bmeng.cn@gmail.com> wrote:
>
> From: Bin Meng <bin.meng@windriver.com>
>
> 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>

This is great! Thanks!

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>
>  docs/system/target-riscv.rst | 62 ++++++++++++++++++++++++++++++++++++
>  docs/system/targets.rst      |  1 +
>  2 files changed, 63 insertions(+)
>  create mode 100644 docs/system/target-riscv.rst
>
> diff --git a/docs/system/target-riscv.rst b/docs/system/target-riscv.rst
> new file mode 100644
> index 0000000000..978b96cbdb
> --- /dev/null
> +++ b/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 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 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
> +-------------------
> diff --git a/docs/system/targets.rst b/docs/system/targets.rst
> index 564cea9a9b..75ed1087fd 100644
> --- a/docs/system/targets.rst
> +++ b/docs/system/targets.rst
> @@ -19,6 +19,7 @@ Contents:
>     target-m68k
>     target-mips
>     target-ppc
> +   target-riscv
>     target-rx
>     target-s390x
>     target-sparc
> --
> 2.25.1
>
>