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Re: [Qemu-devel] [PATCH 2/7] tests/qgraph: pci-pc driver and interface n

From: Emanuele
Subject: Re: [Qemu-devel] [PATCH 2/7] tests/qgraph: pci-pc driver and interface nodes
Date: Wed, 18 Jul 2018 20:29:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 07/18/2018 04:29 PM, Stefan Hajnoczi wrote:
On Wed, Jul 11, 2018 at 05:18:03PM +0200, Paolo Bonzini wrote:
On 11/07/2018 16:49, Stefan Hajnoczi wrote:
On Mon, Jul 09, 2018 at 11:11:31AM +0200, Emanuele Giuseppe Esposito wrote:
-QPCIBus *qpci_init_pc(QTestState *qts, QGuestAllocator *alloc)
+static void *qpci_get_driver(void *obj, const char *interface)
-    QPCIBusPC *ret = g_new0(QPCIBusPC, 1);
+    QPCIBusPC *qpci = obj;
+    if (!g_strcmp0(interface, "pci-bus")) {
+        return &qpci->bus;
+    }
+    printf("%s not present in pci-bus-pc", interface);
+    abort();
At this point I wonder if it makes sense to use the QEMU Object Model
(QOM), which has interfaces and inheritance.  qgraph duplicates part of
the object model.
Replying for Emanuele on this point since we didn't really go through
QOM yet; I'll let him answer the comments that are more related to the code.

QOM is much heavier weight than qgraph, and adds a lot more boilerplate:

- QOM properties interact with QAPI and bring in a lot of baggage;
qgraph would only use "child" properties to implement containment.

- QOM objects are long-lived, qgraph objects only last for the duration
of a single test.  qgraph doesn't need reference counting or complex
two-phase initialization like "realize" or "user_complete"

- QOM's hierarchy is shallow, but qgraph doesn't really need _any_
hierarchy.  Because it focuses on interface rather than hierarchy,
everything can be expressed simply through structs contained into one

Consider that qgraph is 1/4th the size of QOM, and a large part of it is
the graph data structure that (as you said) would not be provided by QOM.

There are two things where using QOM would save a little bit of
duplicated concepts, namely the get_driver (get interface, what you
quote above) and get_device (get contained object) callbacks.  However,
it wouldn't simplify the code at all, because it would require the
introduction of separate instance and class structs.  We didn't want to
add all too much boilerplate for people that want to write qtest, as you
pointed out in the review of patch 4.
Yes, I think these are good points.  QOM does involve a lot of
boilerplate for defining objects.

+void qpci_set_pc(QPCIBusPC *ret, QTestState *qts, QGuestAllocator *alloc)
It's not clear to me what the purpose of this function is - at least the
name is a bit cryptic since it seems more like an initialization
function than 'setting pc' on QPCIBusPC.  How about inlining this in
qpci_init_pc() instead of keeping a separate function?
I agree about the naming.  Perhaps we can rename the existing
qpci_init_pc to qpci_pc_new, and qpci_set_pc to qpci_pc_init.

Would you prefer if the split was done in the patch that introduces the
user for qpci_set_pc (patch 5)?  We did it here because this patch
prepares qpci-pc
Either way is fine.  I suggested inlining mainly because it avoids the
need to pick a good name :).
I had to put this patch here because it also introduces qpci_device_init, used by sdhci (patch 3).

For the next version I plan to have a patch X where I rename all occurrences of qpci_init_pc in qpci_pc_new, and a patch X+1 that introduces qpci_init_pc (was qpci_set_pc) and the other changes.

Should I only introduce qpci_device_init in patch 3 and the remaining things in patch 5?

I think the general problem here is that in some patches I create functions that are planned to only be used only in next patches (of the current series).

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