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Re: [Qemu-devel] [RFC] Plan for moving forward with QOM

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] Plan for moving forward with QOM
Date: Fri, 16 Sep 2011 16:03:42 -0500
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On 09/16/2011 03:48 PM, Gleb Natapov wrote:
On Fri, Sep 16, 2011 at 02:29:51PM -0500, Anthony Liguori wrote:
On 09/16/2011 02:13 PM, Gleb Natapov wrote:
On Fri, Sep 16, 2011 at 01:42:02PM -0500, Anthony Liguori wrote:
On 09/16/2011 01:22 PM, Gleb Natapov wrote:
Then we are arguing about minor detail. But according to you this minor
detail will prevent us from walking device tree up to the root, so it is
not so minor for me.

There is no root.  It's not a tree.  The composition tree (which
There is "virtual root". System bus in qdev speak. You can create one
explicitly like qdev did or you can say that if a device has no parent
it is on a system bus.

Graphs don't have roots.

Lets not pretend that this device graph is absolutely arbitrary.

we've been talking about using for canonical pathnames) has nothing
to do with the buses.

I do not care about canonical pathnames you are talking about too
much. The reason is that they are useless outside of QEMU. The problem
we need to solve is to name a device in such a way that it can be found
without knowing any QEMU implementation details. This is not for internal
QEMU use (for that you can use canonical pathnames you are talking about),
but for communicating device location outside of QEMU. Currently we pass
OF device paths to firmware and this is ABI QEMU expose to a guest, so
QOM needs to preserve it. What I asked is how it can be done with QOM and
you are saying that it can't be done and this is not a very good answer.

No, it's very easy.  Something just has to decide where to start the
transversal, and then walk the graph starting at that node until
they find the node.  They need to map each node to whatever the OF
representation is that makes sense.

That is obvious thing to do, but you said not all devices have a link to
a parent node which will make such traversal impossible.

No, it's really as simple as this:

bool build_of_path(Device *node, Device *target, char *path)
 if (IS_PCI_BUS(node)) {
   for (int i = 0; i < 32; i++) {
     PciBus *bus = PCI_BUS(node);
     if (build_of_path(bus->slot[i], target, path)) {
        strappend(path, "/address@hidden", i);
        return true;
 } else if (IS_ISA_BUS(node)) {
   IsaBus *bus = ISA_BUS(node);
   for (int i = 0; i < bus->nb_slots; i++) {
      if (build_of_path(bus->slots[i], target, path)) {
         strappend(path, "/isa@" TARGET_FMT_plx, bus->slots[i]->mem.base);
         return true;
 } else if (node == target) {
   return true;

 return false;

This uses centralized logic which I think is the right thing to do. You could also have an OpenFirmwareCompatBus interface which implemented this logic within each bus and then a bus could choose to implement this interface. I think centralized is better though.

Another thing
is that graph needs to contain enough information (nodes) to create
correct path. If a bus is omitted resulting path will not be accurate.

It's really not that different from what we have today except that you can interact better with the devices in C. Whereas there is no way today to read the "iobase" property of the IsaSerial device, there would be an isa_serial_get_iobase() in QOM that the property would be automatically implemented in terms of.

A tree is just a degenerate graph so going from QOM to OF is easy.
It just requires looking at a subset.

ABI requires OF path to be built not for all QEMU devices, but only for
those that support bootindex property, so this may make our task more
simple, although I think the correct solution should be generic.

To be fair, it's not an ABI that is supported.  We only need to
support a single BIOS version that we provide.
Yes, but how is this help us here? We will still have to provide some
device identification, that is independent from QEMU internals, to
a firmware.  We chose to use OF because it is established standard and
using something different will be NIH. Seabios is not the only QEMU's
firmware and Seabios is used not only with QEMU (and coreboot is/will be
able to pass bootorder to Seabios the same way QEMU does). So inventing
QEMU specific format, that will have to contain all the same information
about device as OF path anyway, is just spreading NIH around.

No one is suggesting getting rid of OF paths. You're confusing constructing paths with device naming. They are not the same thing.


Anthony Liguori

But that's just splitting hairs.  You can still generate these paths.


Anthony Liguori


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