Re: QMP introspecting device props common to a bus type

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> When introspecting properties for devices, libvirt issues a sequence of
> QMP  'device-list-properties' commands, one for each device type we
> need info for.  The result of this command tells us about all properties
> possible on that specific device, which is generally just fine.
>
> Every now and then though, there are properties that are inherited from
> / defined by the parent class, usually props that are common to all
> devices attached to a given bus type.
>
> The current case in point is the "acpi-index" property that was added to
> the "PCI" bus type, that is a parent for any type that is a PCI dev.
>
> Generally when libvirt adds support for a property, it will enable it
> across all devices that can support the property. So we're enabling use
> of "acpi-index" across all PCI devices.
>
> The question thus becomes how should we probe for existence of the
> "acpi-index" property. The qemu-system-x86_64 emulator has somewhere
> around 150 user creatable PCI devices according to "-device help".
>
> The existance of a class hierarchy is explicitly not exposed in QMP
> because we consider that an internal impl detail, so we can't just
> query "acpi-index" on the "PCI" parent type. 

Not true.

qapi/qom.json:

    ##
    # @ObjectTypeInfo:
    #
    # This structure describes a search result from @qom-list-types
    #
    # @name: the type name found in the search
    #
    # @abstract: the type is abstract and can't be directly instantiated.
    #            Omitted if false. (since 2.10)
    #
    # @parent: Name of parent type, if any (since 2.10)
    #
    # Since: 1.1
    ##
    { 'struct': 'ObjectTypeInfo',
      'data': { 'name': 'str', '*abstract': 'bool', '*parent': 'str' } }

    ##
    # @qom-list-types:
    #
    # This command will return a list of types given search parameters
    #
    # @implements: if specified, only return types that implement this type name
    #
    # @abstract: if true, include abstract types in the results
    #
    # Returns: a list of @ObjectTypeInfo or an empty list if no results are 
found
    #
    # Since: 1.1
    ##
    { 'command': 'qom-list-types',
      'data': { '*implements': 'str', '*abstract': 'bool' },
      'returns': [ 'ObjectTypeInfo' ],
      'allow-preconfig': true }

Example 1:

    {"execute": "qom-list-types", "arguments": {"abstract": true}}

returns all type names with their parent type names.

The following script prints a QOM type forest:

    #!/usr/bin/python3

    true = True
    false = False
    ts = ... output of the qom-list-types above ...
    child={}

    for t in ts:
        n = t['name']
        p = t.get('parent')
        if p not in child:
            child[p] = []
        child[p].append(n)

    def print_type_tree(name, level=-1):
        if name is not None:
            print(" " * level * 4 + name)
        for c in child.get(name, []):
            print_type_tree(c, level + 1)

    print_type_tree(None)

Example 2:

    {"execute": "qom-list-types", "arguments": {"implements": "pci-device"}}

returns all the (concrete) PCI device type names.

Note that "implements" may be an interface, too.

> We certainly don't want to issue 'device-list-properties' over and
> over for all 147 devices.
>
> If we just pick one device type, say virtio-blk-pci, and query that
> for "acpi-index", then our code is fragile because anyone can make
> a QEMU build that compiles-out a specific device. This is fairly
> unlikely for virtio devices, but never say never.
>
> For PCI, i'm tending towards probing for the "acpi-index" property on
> both "pci-bridge" and "pcie-root-port", as it seems unlikely that both
> of those will be compiled out of QEMU while still retaining PCI support.
>
> I'm wondering if QEMU maintainers have a view on "best practice" to
> probe for device props that are common to specific bus types ?

The obvious

    {"execute": "device-list-properties",
     "arguments": {"typename": "pci-device"}}

fails with "Parameter 'typename' expects a non-abstract device type".
But its cousin qom-list-properties works:

    {"execute": "qom-list-properties",
     "arguments": {"typename": "pci-device"}}
    {"return": [
     {"name": "type", "type": "string"},
     {"name": "parent_bus", "type": "link<bus>"},
     {"name": "realized", "type": "bool"},
     {"name": "hotplugged", "type": "bool"},
     {"name": "hotpluggable", "type": "bool"},
     {"name": "failover_pair_id", "type": "str"},
     {"name": "romfile", "type": "str"},
     {"name": "addr", "description": "Slot and optional function number, 
example: 06.0 or 06", "type": "int32"},
     {"name": "romsize", "type": "uint32"},
     {"name": "x-pcie-lnksta-dllla", "description": "on/off", "type": "bool"},
     {"name": "rombar", "type": "uint32"},
     {"name": "x-pcie-extcap-init", "description": "on/off", "type": "bool"},
     {"name": "acpi-index", "type": "uint32"},
     {"name": "multifunction", "description": "on/off", "type": "bool"},
     {"name": "legacy-addr", "type": "str"}]}

Does this help?