Re: [Qemu-devel] [PATCH] hw/arm/virt: explain device-to-transport mapping in create_virtio_devices()

[Laszlo Ersek <lersek@redhat.com>]

Peter,

On 01/30/15 05:31, Laszlo Ersek wrote:
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  hw/arm/virt.c | 32 ++++++++++++++++++++++++++++----
>  1 file changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 2353440..091e5ee 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -441,10 +441,27 @@ static void create_virtio_devices(const VirtBoardInfo *vbi, qemu_irq *pic)
>      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 reserved; see eg.

can you please s/reserved/reversed/?

Result of over-editing, sorry.

Thanks

> +     * 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.
>       */
>      for (i = 0; i < NUM_VIRTIO_TRANSPORTS; i++) {
>          int irq = vbi->irqmap[VIRT_MMIO] + i;
> @@ -453,6 +470,13 @@ static void create_virtio_devices(const VirtBoardInfo *vbi, qemu_irq *pic)
>          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;
>