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Re: [Qemu-devel] [PATCH] initialize unit id of IDE bus


From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH] initialize unit id of IDE bus
Date: Tue, 26 Oct 2010 11:29:49 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

Gleb Natapov <address@hidden> writes:

> On Mon, Oct 25, 2010 at 08:06:13PM +0200, Markus Armbruster wrote:
>> Gleb Natapov <address@hidden> writes:
>> 
>> > On Mon, Oct 25, 2010 at 06:22:12PM +0200, Markus Armbruster wrote:
>> >> Gleb Natapov <address@hidden> writes:
>> >> 
>> >> > On Mon, Oct 25, 2010 at 04:29:35PM +0200, Markus Armbruster wrote:
>> >> >> Gleb Natapov <address@hidden> writes:
>> >> >> 
>> >> >> > Without this patch both buses on PIIX3_IDE device have the same unit 
>> >> >> > id.
>> >> >> 
>> >> >> Are you sure that's wrong?
>> >> >> 
>> >> > So how do I know which bus is it on PIIX3_IDE?
>> >> [...]
>> >> 
>> >> Let me try to explain the IDE pointer thicket.
>> >> 
>> >> piix3-ide provides two IDE buses.  pci_piix_ide_initfn() stores them in
>> >> PCIIDEState member IDEBus bus[2].  Technically redundant, because qdev
>> >> stores child buses in dev.qdev.child_bus.
>> >> 
>> >> IDEBus points back: qbus.parent.
>> >> 
>> >> Up to two IDE devices can sit on each IDE bus.  The first one uses
>> >> IDEBus members master and ifs[0], the second one uses slave and ifs[1].
>> >> 
>> >> ifs[i].bus points back to the IDE bus.
>> >> 
>> >> {master,slave}.qdev.parent_bus point back to the IDE bus.
>> >> 
>> >> Say you got an IDEDevice and want to know which to which of the two
>> >> buses it's connected.  Let's call it d.
>> >> 
>> >> d->qdev.parent_bus is the BusState.
>> >> 
>> >> Upcast to IDEBus: b = DO_UPCAST(IDEBus, qbus, d->qdev.parent_bus).
>> >> 
>> >> b->qbus.parent is the IDE controller.
>> >> 
>> >> Upcast to PCIIDEState: c = DO_UPCAST(PCIIDEState, dev, n->qbus.parent);
>> >> 
>> >> If c->bus[0] == b, it's on the first bus.
>> >> 
>> >> Else it must be on the second bus, i.e. c->bus[1] == b.
>> >> 
>> >> Hope I didn't screw this up too badly.
>> > Will check tomorrow if this works, but why not have simple property on
>> > IDEBus that says which one is it?
>> 
>> Because nobody has needed it so far?
>> 
>> Does the bus care whether it's first or second?
>> 
> User that wants to address particular disk cares.

Define "user".

For users of qemu, the way to address a particular disk (or any qdev
device) is by user-specified ID.

For users within qemu, there are other ways.

I'm asking whether the bus cares, because IDEBus holds the state of the
bus.  If the bus itself doesn't care whether it's primary or secondary,
then this state should not carry that information.

>> >                                   BTW what -device magic should I use to
>> > create secondary disk on second IDE bus?
>> 
>> Try -device ide-drive,bus=ide.1,unit=1,drive=...
>                                                                  Can I

In qdev, we address buses by ID, just like devices.  Unlike device IDs,
which are specified by the user, the bus IDs are auto-generated.  The
secondary bus of piix3-ide gets the bus ID "ide.1".

We do have usability problems there.  There is no way for the user to
specify an ID for a default device, as far as I know.  The bus IDs are
defined by the device providing the bus.  If it provides none, qdev core
makes one up.  This breaks down in corner cases.  For instance, with -M
isapc, we get two isa-ide.  Each provides an IDE bus without specifying
its ID.  qdev core assigns the same name "ide.0" to both.  bus=ide.0
picks the first one (in not-really-specified qtree traversal order).
As far as I can tell, there's no way to address the second one.

>                                                                  Can I
> instantiate ide-device on USB bus by doing -device ide-drive,bus=usb?

-usb -drive if=none,file=tmp.qcow2,id=dr0 \
-device ide-drive,bus=usb.0,unit=1,drive=dr0
qemu-system-x86_64: -device ide-drive,bus=usb.0,unit=1,drive=dr0: Device 
'ide-drive' can't go on a USB bus



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