docs/system: riscv: Add pflash usage details

pflash devices can be used in virt machine for different
purposes like for ROM code or S-mode FW payload. Add a
section in the documentation on how to use pflash devices
for different purposes.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230601045910.18646-4-sunilvl@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/system/riscv/virt.rst

@@ -53,6 +53,37 @@ 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.
 
+Using flash devices
+-------------------
+
+By default, the first flash device (pflash0) is expected to contain
+S-mode firmware code. It can be configured as read-only, with the
+second flash device (pflash1) available to store configuration data.
+
+For example, booting edk2 looks like
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 \
+     -blockdev node-name=pflash0,driver=file,read-only=on,filename=<edk2_code> \
+     -blockdev node-name=pflash1,driver=file,filename=<edk2_vars> \
+     -M virt,pflash0=pflash0,pflash1=pflash1 \
+     ... other args ....
+
+For TCG guests only, it is also possible to boot M-mode firmware from
+the first flash device (pflash0) by additionally passing ``-bios
+none``, as in
+
+.. code-block:: bash
+
+  $ qemu-system-riscv64 \
+     -bios none \
+     -blockdev node-name=pflash0,driver=file,read-only=on,filename=<m_mode_code> \
+     -M virt,pflash0=pflash0 \
+     ... other args ....
+
+Firmware images used for pflash must be exactly 32 MiB in size.
+
 Machine-specific options
 ------------------------