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

[Gleb Natapov <gleb@redhat.com>]

On Tue, Oct 26, 2010 at 11:29:49AM +0200, Markus Armbruster wrote:
> Gleb Natapov <gleb@redhat.com> writes:
> 
> > On Mon, Oct 25, 2010 at 08:06:13PM +0200, Markus Armbruster wrote:
> >> Gleb Natapov <gleb@redhat.com> writes:
> >> 
> >> > On Mon, Oct 25, 2010 at 06:22:12PM +0200, Markus Armbruster wrote:
> >> >> Gleb Natapov <gleb@redhat.com> writes:
> >> >> 
> >> >> > On Mon, Oct 25, 2010 at 04:29:35PM +0200, Markus Armbruster wrote:
> >> >> >> Gleb Natapov <gleb@redhat.com> 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".
> 
User is a guest OS.

> 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.
> 
Which 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.
> 
Bus cares because devices on the bus are addressed differently depending
on which bus it resides on.

> >> >                                   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".
Unfortunately those ids are meaningless from outside of qemu.

> 
> 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.
> 
This looks like fundamental problem in qdev design. In my case bus names
assigned by qdev are useless even when done right though :(

> >                                                                  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

--
			Gleb.