hw/arm/virt: explain device-to-transport mapping in create_virtio_devices()

    int i;

    hwaddr size = vbi->memmap[VIRT_MMIO].size;

    /* Note that we have to create the transports in forwards order

     * so that command line devices are inserted lowest address first,

     * and then add dtb nodes in reverse order so that they appear in

     * the finished device tree lowest address first.

    /* We create the transports in forwards order. Since qbus_realize()

     * prepends (not appends) new child buses, the incrementing loop below will

     * create a list of virtio-mmio buses with decreasing base addresses.

     *

     * When a -device option is processed from the command line,

     * qbus_find_recursive() picks the next free virtio-mmio bus in forwards

     * order. The upshot is that -device options in increasing command line

     * order are mapped to virtio-mmio buses with decreasing base addresses.

     *

     * When this code was originally written, that arrangement ensured that the

     * guest Linux kernel would give the lowest "name" (/dev/vda, eth0, etc) to

     * the first -device on the command line. (The end-to-end order is a

     * function of this loop, qbus_realize(), qbus_find_recursive(), and the

     * guest kernel's name-to-address assignment strategy.)

     *

     * Meanwhile, the kernel's traversal seems to have been reversed; see eg.

     * the message, if not necessarily the code, of commit 70161ff336.

     * Therefore the loop now establishes the inverse of the original intent.

     *

     * Unfortunately, we can't counteract the kernel change by reversing the

     * loop; it would break existing command lines.

     *

     * In any case, the kernel makes no guarantee about the stability of

     * enumeration order of virtio devices (as demonstrated by it changing

     * between kernel versions). For reliable and stable identification

     * of disks users must use UUIDs or similar mechanisms.

     */

    for (i = 0; i < NUM_VIRTIO_TRANSPORTS; i++) {

        int irq = vbi->irqmap[VIRT_MMIO] + i;

        sysbus_create_simple("virtio-mmio", base, pic[irq]);

    }

    /* We add dtb nodes in reverse order so that they appear in the finished

     * device tree lowest address first.

     *

     * Note that this mapping is independent of the loop above. The previous

     * loop influences virtio device to virtio transport assignment, whereas

     * this loop controls how virtio transports are laid out in the dtb.

     */

    for (i = NUM_VIRTIO_TRANSPORTS - 1; i >= 0; i--) {

        char *nodename;

        int irq = vbi->irqmap[VIRT_MMIO] + i;