[PATCH 2/2] docs/system: riscv: Add documentation for virt machine

[Bin Meng <bmeng.cn@gmail.com>]

This adds detailed documentation for RISC-V `virt` machine,
including the following information:

  - Supported devices
  - Hardware configuration information
  - Boot options
  - Running Linux kernel
  - Running U-Boot

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
---

 docs/system/riscv/virt.rst   | 138 +++++++++++++++++++++++++++++++++++
 docs/system/target-riscv.rst |   1 +
 2 files changed, 139 insertions(+)
 create mode 100644 docs/system/riscv/virt.rst

diff --git a/docs/system/riscv/virt.rst b/docs/system/riscv/virt.rst
new file mode 100644
index 0000000000..3709f05797
--- /dev/null
+++ b/docs/system/riscv/virt.rst
@@ -0,0 +1,138 @@
+'virt' Generic Virtual Platform (``virt``)
+==========================================
+
+The `virt` board is a platform which does not correspond to any real hardware;
+it is designed for use in virtual machines. It is the recommended board type
+if you simply want to run a guest such as Linux and do not care about
+reproducing the idiosyncrasies and limitations of a particular bit of
+real-world hardware.
+
+Supported devices
+-----------------
+
+The ``virt`` machine supports the following devices:
+
+* Up to 8 generic RV32GC/RV64GC cores, with optional extensions
+* Core Local Interruptor (CLINT)
+* Platform-Level Interrupt Controller (PLIC)
+* CFI parallel NOR flash memory
+* 1 NS16550 compatible UART
+* 1 Google Goldfish RTC
+* 1 SiFive Test device
+* 8 virtio-mmio transport devices
+* 1 generic PCIe host bridge
+* The fw_cfg device that allows a guest to obtain data from QEMU
+
+Note that the default CPU is a generic RV32GC/RV64GC. Optional extensions
+can be enabled via command line parameters, e.g.: ``-cpu rv64,x-h=true``
+enables the hypervisor extension for RV64.
+
+Hardware configuration information
+----------------------------------
+
+The ``virt`` machine automatically generates a device tree blob ("dtb")
+which it passes to the guest, if there is no ``-dtb`` option. This provides
+information about the addresses, interrupt lines and other configuration of
+the various devices in the system. Guest software should discover the devices
+that are present in the generated DTB.
+
+If users want to provide their own DTB, they can use the ``-dtb`` option.
+These DTBs should have the following requirements:
+
+* The number of subnodes of the /cpus node should match QEMU's ``-smp`` option
+* The /memory reg size should match QEMU’s selected ram_size via ``-m``
+* Should contain a node for the CLINT device with a compatible string
+  "riscv,clint0" if using with OpenSBI BIOS images
+
+Boot options
+------------
+
+The ``virt`` machine can start using the standard -kernel functionality
+for loading a Linux kernel, a VxWorks kernel, an S-mode U-Boot bootloader
+with the default OpenSBI firmware image as the -bios. It also supports
+the recommended RISC-V bootflow: U-Boot SPL (M-mode) loads OpenSBI fw_dynamic
+firmware and U-Boot proper (S-mode), using the standard -bios functionality.
+
+Running Linux kernel
+--------------------
+
+Linux mainline v5.12 release is tested at the time of writing. To build a
+Linux mainline kernel that can be booted by the ``virt`` machine in
+64-bit mode, simply configure the kernel using the defconfig configuration:
+
+.. code-block:: bash
+
+  $ export ARCH=riscv
+  $ export CROSS_COMPILE=riscv64-linux-
+  $ make defconfig
+  $ make
+
+To boot the newly built Linux kernel in QEMU with the ``virt`` machine:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M virt -smp 4 -m 2G \
+      -display none -serial stdio \
+      -kernel arch/riscv/boot/Image \
+      -initrd /path/to/rootfs.cpio \
+      -append "root=/dev/ram"
+
+To build a Linux mainline kernel that can be booted by the ``virt`` machine
+in 32-bit mode, use the rv32_defconfig configuration. A patch is required to
+fix the 32-bit boot issue for Linux kernel v5.12.
+
+.. code-block:: bash
+
+  $ export ARCH=riscv
+  $ export CROSS_COMPILE=riscv64-linux-
+  $ curl https://patchwork.kernel.org/project/linux-riscv/patch/20210627135117.28641-1-bmeng.cn@gmail.com/mbox/ > riscv.patch
+  $ git am riscv.patch
+  $ make rv32_defconfig
+  $ make
+
+Replace ``qemu-system-riscv64`` with ``qemu-system-riscv32`` in the command
+line above to boot the 32-bit Linux kernel. A rootfs image containing 32-bit
+applications shall be used in order for kernel to boot to user space.
+
+Running U-Boot
+--------------
+
+U-Boot mainline v2021.04 release is tested at the time of writing. To build an
+S-mode U-Boot bootloader that can be booted by the ``virt`` machine, use
+the qemu-riscv64_smode_defconfig with similar commands as described above for Linux:
+
+.. code-block:: bash
+
+  $ export CROSS_COMPILE=riscv64-linux-
+  $ make qemu-riscv64_smode_defconfig
+
+Boot the 64-bit U-Boot S-mode image directly:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M virt -smp 4 -m 2G \
+      -display none -serial stdio \
+      -kernel /path/to/u-boot.bin
+
+To test booting U-Boot SPL which in M-mode, which in turn loads a FIT image
+that bundles OpenSBI fw_dynamic firmware and U-Boot proper (S-mode) together,
+build the U-Boot images using riscv64_spl_defconfig:
+
+.. code-block:: bash
+
+  $ export CROSS_COMPILE=riscv64-linux-
+  $ export OPENSBI=/path/to/opensbi-riscv64-generic-fw_dynamic.bin
+  $ make qemu-riscv64_spl_defconfig
+
+The minimal QEMU commands to run U-Boot SPL are:
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 -M virt -smp 4 -m 2G \
+      -display none -serial stdio \
+      -bios /path/to/u-boot-spl \
+      -device loader,file=/path/to/u-boot.itb,addr=0x80200000
+
+To test 32-bit U-Boot images, switch to use qemu-riscv32_smode_defconfig and
+riscv32_spl_defconfig builds, and replace ``qemu-system-riscv64`` with
+``qemu-system-riscv32`` in the command lines above to boot the 32-bit U-Boot.
diff --git a/docs/system/target-riscv.rst b/docs/system/target-riscv.rst
index a5cc06b726..89a866e4f4 100644
--- a/docs/system/target-riscv.rst
+++ b/docs/system/target-riscv.rst
@@ -69,6 +69,7 @@ undocumented; you can get a complete list by running
    riscv/microchip-icicle-kit
    riscv/shakti-c
    riscv/sifive_u
+   riscv/virt
 
 RISC-V CPU firmware
 -------------------
-- 
2.25.1