target/loongarch: Add system emulation introduction

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220606124333.2060567-19-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

docs/system/loongarch/loongson3.rst

@@ -0,0 +1,41 @@
+:orphan:
+
+==========================================
+loongson3 virt generic platform (``virt``)
+==========================================
+
+The ``virt`` machine use gpex host bridge, and there are some
+emulated devices on virt board, such as loongson7a RTC device,
+IOAPIC device, ACPI device and so on.
+
+Supported devices
+-----------------
+
+The ``virt`` machine supports:
+- Gpex host bridge
+- Ls7a RTC device
+- Ls7a IOAPIC device
+- Ls7a ACPI device
+- Fw_cfg device
+- PCI/PCIe devices
+- Memory device
+- CPU device. Type: Loongson-3A5000.
+
+CPU and machine Type
+--------------------
+
+The ``qemu-system-loongarch64`` provides emulation for virt
+machine. You can specify the machine type ``virt`` and
+cpu type ``Loongson-3A5000``.
+
+Boot options
+------------
+
+Now the ``virt`` machine can run test program in ELF format and the
+method of compiling is in target/loongarch/README.
+
+.. code-block:: bash
+
+  $ qemu-system-loongarch64 -machine virt -m 4G -cpu Loongson-3A5000 \
+      -smp 1 -kernel hello -monitor none -display none \
+      -chardev file,path=hello.out,id=output -serial chardev:output