riscv: Add QEMU virt board support

This adds QEMU RISC-V 'virt' board target support, with the hope of
helping people easily test U-Boot on RISC-V.

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, 16550A UART devices in addition to VirtIO and
it also uses device-tree to pass configuration information to guest
software. It implements RISC-V privileged architecture spec v1.10.

Both 32-bit and 64-bit builds are supported. Support is pretty much
preliminary, only booting to U-Boot shell with the UART driver on
a single core. Booting Linux is not supported yet.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>

1 files changed, 46 insertions, 0 deletions

diff --git a/doc/README.qemu-riscv b/doc/README.qemu-riscv
new file mode 100644
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+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
+
+U-Boot on QEMU's 'virt' machine on 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.
+
+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,
+16550A UART devices in addition to VirtIO and it also uses device-tree to pass
+configuration information to guest software. It implements RISC-V privileged
+architecture spec v1.10.
+
+Building U-Boot
+---------------
+Set the CROSS_COMPILE environment variable as usual, and run:
+
+- For 32-bit RISC-V:
+    make qemu-riscv32_defconfig
+    make
+
+- For 64-bit RISC-V:
+    make qemu-riscv64_defconfig
+    make
+
+Running U-Boot
+--------------
+The minimal QEMU command line to get U-Boot up and running is:
+
+- For 32-bit RISC-V:
+    qemu-system-riscv32 -nographic -machine virt -kernel u-boot
+
+- For 64-bit RISC-V:
+    qemu-system-riscv64 -nographic -machine virt -kernel u-boot
+
+The commands above create targets with 128MiB memory by default.
+A freely configurable amount of RAM can be created via the '-m'
+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.
+
+These have been tested in QEMU 3.0.0.