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+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
+
+VirtIO Support
+==============
+
+This document describes the information about U-Boot support for VirtIO [1]
+devices, including supported boards, build instructions, driver details etc.
+
+What's VirtIO?
+--------------
+VirtIO is a virtualization standard for network and disk device drivers where
+just the guest's device driver "knows" it is running in a virtual environment,
+and cooperates with the hypervisor. This enables guests to get high performance
+network and disk operations, and gives most of the performance benefits of
+paravirtualization. In the U-Boot case, the guest is U-Boot itself, while the
+virtual environment are normally QEMU [2] targets like ARM, RISC-V and x86.
+
+Status
+------
+VirtIO can use various different buses, aka transports as described in the
+spec. While VirtIO devices are commonly implemented as PCI devices on x86,
+embedded devices models like ARM/RISC-V, which does not normally come with
+PCI support might use simple memory mapped device (MMIO) instead of the PCI
+device. The memory mapped virtio device behaviour is based on the PCI device
+specification. Therefore most operations including device initialization,
+queues configuration and buffer transfers are nearly identical. Both MMIO
+and PCI transport options are supported in U-Boot.
+
+The VirtIO spec defines a lots of VirtIO device types, however at present only
+network and block device, the most two commonly used devices, are supported.
+
+The following QEMU targets are supported.
+
+  - qemu_arm_defconfig
+  - qemu_arm64_defconfig
+  - qemu-riscv32_defconfig
+  - qemu-riscv64_defconfig
+  - qemu-x86_defconfig
+  - qemu-x86_64_defconfig
+
+Note ARM and RISC-V targets are configured with VirtIO MMIO transport driver,
+and on x86 it's the PCI transport driver.
+
+Build Instructions
+------------------
+Building U-Boot for pre-configured QEMU targets is no different from others.
+For example, we can do the following with the CROSS_COMPILE environment
+variable being properly set to a working toolchain for ARM:
+
+  $ make qemu_arm_defconfig
+  $ make
+
+You can even create a QEMU ARM target with VirtIO devices showing up on both
+MMIO and PCI buses. In this case, you can enable the PCI transport driver
+from 'make menuconfig':
+
+Device Drivers  --->
+	...
+	VirtIO Drivers  --->
+		...
+		[*] PCI driver for virtio devices
+
+Other drivers are at the same location and can be tuned to suit the needs.
+
+Requirements
+------------
+It is required that QEMU v2.5.0+ should be used to test U-Boot VirtIO support
+on QEMU ARM and x86, and v2.12.0+ on QEMU RISC-V.
+
+Testing
+-------
+The following QEMU command line is used to get U-Boot up and running with
+VirtIO net and block devices on ARM.
+
+  $ qemu-system-arm -nographic -machine virt -bios u-boot.bin \
+    -netdev tap,ifname=tap0,id=net0 \
+    -device virtio-net-device,netdev=net0 \
+    -drive if=none,file=test.img,format=raw,id=hd0 \
+    -device virtio-blk-device,drive=hd0
+
+On x86, command is slightly different to create PCI VirtIO devices.
+
+  $ qemu-system-i386 -nographic -bios u-boot.rom \
+    -netdev tap,ifname=tap0,id=net0 \
+    -device virtio-net-pci,netdev=net0 \
+    -drive if=none,file=test.img,format=raw,id=hd0 \
+    -device virtio-blk-pci,drive=hd0
+
+Additional net and block devices can be created by appending more '-device'
+parameters. It is also possible to specify both MMIO and PCI VirtIO devices.
+For example, the following commnad creates 3 VirtIO devices, with 1 on MMIO
+and 2 on PCI bus.
+
+  $ qemu-system-arm -nographic -machine virt -bios u-boot.bin \
+    -netdev tap,ifname=tap0,id=net0 \
+    -device virtio-net-pci,netdev=net0 \
+    -drive if=none,file=test0.img,format=raw,id=hd0 \
+    -device virtio-blk-device,drive=hd0 \
+    -drive if=none,file=test1.img,format=raw,id=hd1 \
+    -device virtio-blk-pci,drive=hd1
+
+By default QEMU creates VirtIO legacy devices by default. To create non-legacy
+(aka modern) devices, pass additional device property/value pairs like below:
+
+  $ qemu-system-i386 -nographic -bios u-boot.rom \
+    -netdev tap,ifname=tap0,id=net0 \
+    -device virtio-net-pci,netdev=net0,disable-legacy=true,disable-modern=false \
+    -drive if=none,file=test.img,format=raw,id=hd0 \
+    -device virtio-blk-pci,drive=hd0,disable-legacy=true,disable-modern=false
+
+A 'virtio' command is provided in U-Boot shell.
+
+  => virtio
+  virtio - virtio block devices sub-system
+
+  Usage:
+  virtio scan - initialize virtio bus
+  virtio info - show all available virtio block devices
+  virtio device [dev] - show or set current virtio block device
+  virtio part [dev] - print partition table of one or all virtio block devices
+  virtio read addr blk# cnt - read `cnt' blocks starting at block
+       `blk#' to memory address `addr'
+  virtio write addr blk# cnt - write `cnt' blocks starting at block
+       `blk#' from memory address `addr'
+
+To probe all the VirtIO devices, type:
+
+  => virtio scan
+
+Then we can show the connected block device details by:
+
+  => virtio info
+  Device 0: QEMU VirtIO Block Device
+              Type: Hard Disk
+              Capacity: 4096.0 MB = 4.0 GB (8388608 x 512)
+
+And list the directories and files on the disk by:
+
+  => ls virtio 0 /
+  <DIR>       4096 .
+  <DIR>       4096 ..
+  <DIR>      16384 lost+found
+  <DIR>       4096 dev
+  <DIR>       4096 proc
+  <DIR>       4096 sys
+  <DIR>       4096 var
+  <DIR>       4096 etc
+  <DIR>       4096 usr
+  <SYM>          7 bin
+  <SYM>          8 sbin
+  <SYM>          7 lib
+  <SYM>          9 lib64
+  <DIR>       4096 run
+  <DIR>       4096 boot
+  <DIR>       4096 home
+  <DIR>       4096 media
+  <DIR>       4096 mnt
+  <DIR>       4096 opt
+  <DIR>       4096 root
+  <DIR>       4096 srv
+  <DIR>       4096 tmp
+                 0 .autorelabel
+
+Driver Internals
+----------------
+There are 3 level of drivers in the VirtIO driver family.
+
+	+---------------------------------------+
+	|	 virtio device drivers		|
+	|    +-------------+ +------------+	|
+	|    | virtio-net  | | virtio-blk |	|
+	|    +-------------+ +------------+	|
+	+---------------------------------------+
+	+---------------------------------------+
+	|	virtio transport drivers	|
+	|    +-------------+ +------------+	|
+	|    | virtio-mmio | | virtio-pci |	|
+	|    +-------------+ +------------+	|
+	+---------------------------------------+
+		+----------------------+
+		| virtio uclass driver |
+		+----------------------+
+
+The root one is the virtio uclass driver (virtio-uclass.c), which does lots of
+common stuff for the transport drivers (virtio_mmio.c, virtio_pci.c). The real
+virtio device is discovered in the transport driver's probe() method, and its
+device ID is saved in the virtio uclass's private data of the transport device.
+Then in the virtio uclass's post_probe() method, the real virtio device driver
+(virtio_net.c, virtio_blk.c) is bound if there is a match on the device ID.
+
+The child_post_bind(), child_pre_probe() and child_post_probe() methods of the
+virtio uclass driver help bring the virtio device driver online. They do things
+like acknowledging device, feature negotiation, etc, which are really common
+for all virtio devices.
+
+The transport drivers provide a set of ops (struct dm_virtio_ops) for the real
+virtio device driver to call. These ops APIs's parameter is designed to remind
+the caller to pass the correct 'struct udevice' id of the virtio device, eg:
+
+int virtio_get_status(struct udevice *vdev, u8 *status)
+
+So the parameter 'vdev' indicates the device should be the real virtio device.
+But we also have an API like:
+
+struct virtqueue *vring_create_virtqueue(unsigned int index, unsigned int num,
+					 unsigned int vring_align,
+					 struct udevice *udev)
+
+Here the parameter 'udev' indicates the device should be the transport device.
+Similar naming is applied in other functions that are even not APIs, eg:
+
+static int virtio_uclass_post_probe(struct udevice *udev)
+static int virtio_uclass_child_pre_probe(struct udevice *vdev)
+
+So it's easy to tell which device these functions are operating on.
+
+Development Flow
+----------------
+At present only VirtIO network card (device ID 1) and block device (device
+ID 2) are supported. If you want to develop new driver for new devices,
+please follow the guideline below.
+
+1. add new device ID in virtio.h
+#define VIRTIO_ID_XXX		X
+
+2. update VIRTIO_ID_MAX_NUM to be the largest device ID plus 1
+
+3. add new driver name string in virtio.h
+#define VIRTIO_XXX_DRV_NAME	"virtio-xxx"
+
+4. create a new driver with name set to the name string above
+U_BOOT_DRIVER(virtio_xxx) = {
+	.name = VIRTIO_XXX_DRV_NAME,
+	...
+}
+
+5. provide bind() method in the driver, where virtio_driver_features_init()
+   should be called for driver to negotiate feature support with the device.
+
+6. do funny stuff with the driver
+
+References
+----------
+[1] http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.pdf
+[2] https://www.qemu.org
