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Re: [Qemu-devel] [PATCH] hw/arm/virt: explain device-to-transport mappin
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH] hw/arm/virt: explain device-to-transport mapping in create_virtio_devices() |
Date: |
Tue, 03 Feb 2015 13:04:32 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 |
On 02/03/15 12:51, Peter Maydell wrote:
> On 30 January 2015 at 04:34, Laszlo Ersek <address@hidden> wrote:
>> Peter,
>>
>> On 01/30/15 05:31, Laszlo Ersek wrote:
>>> Signed-off-by: Laszlo Ersek <address@hidden>
>>> ---
>>> 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 address@hidden: note about UUIDs]
betwixt two other tags?
Thanks
Laszlo
PS: yes, I love "betwixt". Whether I used it correctly above is
secondary. ;)