Re: [Qemu-devel] [RFC PATCH 1/5] qdev: Create qdev_get_dev_path()

[Jan Kiszka <jan.kiszka@siemens.com>]

Alex Williamson wrote:
> On Mon, 2010-06-14 at 08:39 +0200, Markus Armbruster wrote:
>> Alex Williamson <alex.williamson@redhat.com> writes:
>>
>>> qdev_get_dev_path() is intended to be the canonical utility for creating
>>> a string representing the qdev hierarchy of a device.  The path consists
>>> of bus and device names as well as identified properties of the immediate
>>> parent bus and device.  This results in strings like:
>>>
>>> "/main-system-bus/pci.0,addr=00.0/i440FX"
>>> "/main-system-bus/pci.0,addr=01.0/PIIX3"
>>> "/main-system-bus/pci.0,addr=02.0/cirrus-vga"
>>> "/main-system-bus/pci.0/isa.0/mc146818rtc"
>>> "/main-system-bus/pci.0/isa.0/isa-serial"
>>> "/main-system-bus/pci.0/isa.0/i8042"
>>> "/main-system-bus/pci.0/isa.0/isa-fdc"
>>> "/main-system-bus/pci.0,addr=03.0/i82551,mac=52:54:00:12:34:56"
>>> "/main-system-bus/pci.0,addr=04.0/virtio-net-pci,mac=52:54:00:12:34:57"
>>> "/main-system-bus/pci.0,addr=05.0/e1000,mac=52:54:00:12:34:58"
>>> "/main-system-bus/pci.0,addr=06.0/rtl8139,mac=52:54:00:12:34:59"
>>> "/main-system-bus/pci.0,addr=07.0/pcnet,mac=52:54:00:12:34:5a"
>>> "/main-system-bus/pci.0,addr=01.1/piix3-ide"
>>> "/main-system-bus/pci.0,addr=01.3/PIIX4_PM"
>>> "/main-system-bus/pci.0,addr=08.0/lsi53c895a"
>>> "/main-system-bus/pci.0,addr=09.0/virtio-blk-pci"
>>>
>>> There are two primary targets for these strings.  The first is vmsave, where
>>> we currently use a device provided string plus instance number to track
>>> SaveStateEntries.  This fails when we introduce device hotplug, particularly
>>> in a case were we have gaps in the instance numbers that cannot easily be
>>> reproduced on a migration target.  The second is for naming RAMBlocks.  For
>>> these, we would like a string that matches the vmstate string.
>> Could you explain why you add "identified properties of the immediate
>> parent bus and device"?  They make the result ver much *not* a "dev
>> path" in the qdev sense...
> 
> In order to try to get a unique string.  Without looking into device
> properties, two e1000s would both be:
> 
> /main-system-bus/pci.0/e1000
> /main-system-bus/pci.0/e1000
> 
> Which is no better than simply "e1000" and would require us to fall back
> to instance ids again.  The goal here is that anything that makes use of
> passing a dev when registering a vmstate gets an instance id of zero.

Will soon (re-)post a patch that adds per-bus instance numbers to deal
with identically named devices. That's required as not all buses have
canonical node IDs (e.g. ISA or the main system bus).

Jan

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