doc: update QEMU RISC-V documentation

The available defconfigs for RISC-V QEMU have changed. We now have
configurations to compile U-Boot to run in supervisor mode and for
U-Boot SPL. Update the QEMU RISC-V documentation to reflect these
changes.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

doc/board/emulation/qemu-riscv.rst

@@ -6,7 +6,8 @@ QEMU RISC-V
 
 QEMU for RISC-V supports a special 'virt' machine designed for emulation and
 virtualization purposes. This document describes how to run U-Boot under it.
-Both 32-bit 64-bit targets are supported.
+Both 32-bit and 64-bit targets are supported, running in either machine or
+supervisor mode.
 
 The QEMU virt machine models a generic RISC-V virtual machine with support for
 the VirtIO standard networking and block storage devices. It has CLINT, PLIC,
@@ -28,6 +29,11 @@ Set the CROSS_COMPILE environment variable as usual, and run:
     make qemu-riscv64_defconfig
     make
 
+This will compile U-Boot for machine mode. To build supervisor mode binaries,
+use the configurations qemu-riscv32_smode_defconfig and
+qemu-riscv64_smode_defconfig instead. Note that U-Boot running in supervisor
+mode requires a supervisor binary interface (SBI), such as RISC-V OpenSBI.
+
 Running U-Boot
 --------------
 The minimal QEMU command line to get U-Boot up and running is:
@@ -46,4 +52,56 @@ parameter. For example, '-m 2G' creates 2GiB memory for the target,
 and the memory node in the embedded DTB created by QEMU reflects
 the new setting.
 
+For instructions on how to run U-Boot in supervisor mode on QEMU
+with OpenSBI, see the documentation available with OpenSBI:
+https://github.com/riscv/opensbi/blob/master/docs/platform/qemu_virt.md
+
 These have been tested in QEMU 3.0.0.
+
+Running U-Boot SPL
+------------------
+In the default SPL configuration, U-Boot SPL starts in machine mode. U-Boot
+proper and OpenSBI (FW_DYNAMIC firmware) are bundled as FIT image and made
+available to U-Boot SPL. Both are then loaded by U-Boot SPL and the location
+of U-Boot proper is passed to OpenSBI. After initialization, U-Boot proper is
+started in supervisor mode by OpenSBI.
+
+OpenSBI must be compiled before compiling U-Boot. Version 0.4 and higher is
+supported by U-Boot. Clone the OpenSBI repository and run the following command.
+
+.. code-block:: console
+
+    git clone https://github.com/riscv/opensbi.git
+    cd opensbi
+    make PLATFORM=qemu/virt
+
+See the OpenSBI documentation for full details:
+https://github.com/riscv/opensbi/blob/master/docs/platform/qemu_virt.md
+
+To make the FW_DYNAMIC binary (build/platform/qemu/virt/firmware/fw_dynamic.bin)
+available to U-Boot, either copy it into the U-Boot root directory or specify
+its location with the OPENSBI environment variable. Afterwards, compile U-Boot
+with the following commands.
+
+- For 32-bit RISC-V::
+
+    make qemu-riscv32_spl_defconfig
+    make
+
+- For 64-bit RISC-V::
+
+    make qemu-riscv64_spl_defconfig
+    make
+
+The minimal QEMU commands to run U-Boot SPL in both 32-bit and 64-bit
+configurations are:
+
+- For 32-bit RISC-V::
+
+    qemu-system-riscv32 -nographic -machine virt -kernel spl/u-boot-spl \
+    -device loader,file=u-boot.itb,addr=0x80200000
+
+- For 64-bit RISC-V::
+
+    qemu-system-riscv64 -nographic -machine virt -kernel spl/u-boot-spl \
+    -device loader,file=u-boot.itb,addr=0x80200000