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>

MAINTAINERS

@@ -1122,6 +1122,14 @@ F: include/hw/net/lasi_82596.h
 F: include/hw/pci-host/dino.h
 F: pc-bios/hppa-firmware.img
 
+LoongArch Machines
+------------------
+Virt
+M: Xiaojuan Yang <yangxiaojuan@loongson.cn>
+M: Song Gao <gaosong@loongson.cn>
+S: Maintained
+F: docs/system/loongarch/loongson3.rst
+
 M68K Machines
 -------------
 an5206

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

target/loongarch/README

@@ -8,3 +8,57 @@
 
   We can get the latest loongarch documents at https://github.com/loongson/LoongArch-Documentation/tags.
 
+
+- System emulation
+
+  Mainly emulate a virt 3A5000 board and ls7a bridge that is not exactly the same as the host.
+  3A5000 support multiple interrupt cascading while here we just emulate the extioi interrupt
+  cascading. LS7A1000 host bridge support multiple devices, such as sata, gmac, uart, rtc
+  and so on. But we just realize the rtc. Others use the qemu common devices. It does not affect
+  the general use. We also introduced the emulation of devices at docs/system/loongarch/loongson3.rst.
+
+  This version only supports running binary files in ELF format, and does not depend on BIOS and kernel file.
+  You can compile the test program with 'make & make check-tcg' and run the test case with the following command:
+
+  1. Install LoongArch cross-tools on X86 machines.
+
+    Download cross-tools.
+
+      wget https://github.com/loongson/build-tools/releases/latest/download/loongarch64-clfs-20211202-cross-tools.tar.xz
+
+      tar -vxf loongarch64-clfs-20211202-cross-tools.tar.xz -C /opt
+
+    Config cross-tools env.
+
+      . setenv.sh
+
+      setenv.sh:
+
+          #!/bin/sh
+          set -x
+          CC_PREFIX=/opt/cross-tools
+
+          export PATH=$CC_PREFIX/bin:$PATH
+          export LD_LIBRARY_PATH=$CC_PREFIX/lib:$LD_LIBRARY_PATH
+          export LD_LIBRARY_PATH=$CC_PREFIX/loongarch64-unknown-linux-gnu/lib/:$LD_LIBRARY_PATH
+          set +x
+
+  2. Test tests/tcg/multiarch.
+
+    ./configure --disable-rdma --disable-pvrdma --prefix=/usr  \
+            --target-list="loongarch64-softmmu"  \
+            --disable-libiscsi --disable-libnfs --disable-libpmem \
+            --disable-glusterfs --enable-libusb --enable-usb-redir \
+            --disable-opengl --disable-xen --enable-spice --disable-werror \
+            --enable-debug --disable-capstone --disable-kvm --enable-profiler
+
+    cd  build/
+
+    make && make check-tcg
+
+    or
+
+    ./build/qemu-system-loongarch64 -machine virt -m 4G -cpu Loongson-3A5000 -smp 1 -kernel build/tests/tcg/loongarch64-softmmu/hello -monitor none -display none -chardev file,path=hello.out,id=output -serial chardev:output
+
+- Note.
+  We can get the latest LoongArch documents or LoongArch tools at https://github.com/loongson/