Steven Sistare <[email protected]> writes:
> On 4/9/2025 9:34 AM, Markus Armbruster wrote:
>> Steven Sistare <[email protected]> writes:
>>> On 4/9/2025 3:39 AM, Markus Armbruster wrote:
>>>> Hi Steve, I apologize for the slow response.
>>>>
>>>> Steve Sistare <[email protected]> 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!
