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

[Laszlo Ersek <lersek@redhat.com>]

On 02/03/15 12:51, Peter Maydell wrote:
> On 30 January 2015 at 04:34, Laszlo Ersek <lersek@redhat.com> wrote:
>> 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.
> 
> Sure, no problem. I also suggest I add this para:
>  *
>  * 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.

Thanks. Do you want me to resubmit, or can you just squash it and
observe that fact in a [peter.maydell@linaro.org: note about UUIDs]
betwixt two other tags?

Thanks
Laszlo
PS: yes, I love "betwixt". Whether I used it correctly above is
secondary. ;)