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

[Alex Williamson <alex.williamson@redhat.com>]

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.

Alex