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

[Peter Maydell <peter.maydell@linaro.org>]

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.

-- PMM