9:44 a.m.
Steven Sistare <steven.sistare(a)oracle.com> writes:
On 4/9/2025 9:34 AM, Markus Armbruster wrote:
> Steven Sistare <steven.sistare(a)oracle.com> writes:
>> On 4/9/2025 3:39 AM, Markus Armbruster wrote:
>>> Hi Steve, I apologize for the slow response.
>>>
>>> Steve Sistare <steven.sistare(a)oracle.com> writes:
>>>
>>>> Using qom-list and qom-get to get all the nodes and property values in a
>>>> QOM tree can take multiple seconds because it requires 1000's of
individual
>>>> QOM requests. Some managers fetch the entire tree or a large subset
>>>> of it when starting a new VM, and this cost is a substantial fraction of
>>>> start up time.
>>>
>>> "Some managers"... could you name one?
>>
>> My personal experience is with Oracle's OCI, but likely others could
benefit.
>
> Peter Krempa tells us libvirt would benefit.
>
>>>> To reduce this cost, consider QAPI calls that fetch more information in
>>>> each call:
>>>> * qom-list-get: given a path, return a list of properties and
values.
>>>> * qom-list-getv: given a list of paths, return a list of properties
and
>>>> values for each path.
>>>> * qom-tree-get: given a path, return all descendant nodes rooted at
that
>>>> path, with properties and values for each.
>>>
>>> Libvirt developers, would you be interested in any of these?
>>>
>>>> In all cases, a returned property is represented by ObjectPropertyValue,
>>>> with fields name, type, value, and error. If an error occurs when
reading
>>>> a value, the value field is omitted, and the error message is returned in
the
>>>> the error field. Thus an error for one property will not cause a bulk
fetch
>>>> operation to fail.
>>>
>>> Returning errors this way is highly unusual. Observation; I'm not
>>> rejecting this out of hand. Can you elaborate a bit on why it's useful?
>>
>> It is considered an error to read some properties if they are not valid for
>> the configuration. And some properties are write-only and return an error
>> if they are read. Examples:
>>
>> legacy-i8042: <EXCEPTION: Property 'vmmouse.legacy-i8042' is not
readable> (str)
>> legacy-memory: <EXCEPTION: Property
'qemu64-x86_64-cpu.legacy-memory' is not readable> (str)
>> crash-information: <EXCEPTION: No crash occurred>
(GuestPanicInformation)
>>
>> With conventional error handling, if any of these poison pills falls in the
>> scope of a bulk get operation, the entire operation fails.
>
> I suspect many of these poison pills are design mistakes.
>
> If a property is not valid for the configuration, why does it exist?
> QOM is by design dynamic. I wish it wasn't, but as long as it is
> dynamic, I can't see why we should create properties we know to be
> unusable.
>
> Why is reading crash-information an error when no crash occured? This
> is the *normal* case. Errors are for the abnormal.
>
> Anyway, asking you to fix design mistakes all over the place wouldn't be
> fair. So I'm asking you something else instead: do you actually need
> the error information?
I don't need the specific error message.
I could return a boolean meaning "property not available" instead of returning
the exact error message, as long as folks are OK with the output of the qom-tree
script changing for these properties.
Let's put aside the qom-tree script for a moment.
In your patches, the queries return an object's properties as a list of
ObjectPropertyValue, defined as
{ 'struct': 'ObjectPropertyValue',
'data': { 'name': 'str',
'type': 'str',
'*value': 'any',
'*error': 'str' } }
As far as I understand, exactly one of @value and @error are present.
The list has no duplicates, i.e. no two elements have the same value of
"name".
Say we're interested in property "foo". Three cases:
* The list has an element with "name": "foo", and the element has
member
"value": the property exists and "value" has its value.
* The list has an element with "name": "foo", and the element does
not
have member "value": the property exists, but its value cannot be
gotten; member "error" has the error message.
* The list has no element with "name": "foo": the property does not
exist.
If we simply drop ObjectPropertyValue member @error, we lose 'member
"error" has the error message'. That's all.
If a need for more error information should arise later, we could add
member @error. Or something else entirely. Or tell people to qom-get
any properties qom-tree-get couldn't get for error information. My
point is: dropping @error now does not tie our hands as far as I can
tell.
Back to qom-tree. I believe this script is a development aid that
exists because qom-get is painful to use for humans. Your qom-tree
command would completely obsolete it. I wouldn't worry about it.
If you think I'm wrong there, please speak up!