Re: [PATCH V1 0/6] fast qom tree get

[Steven Sistare <steven.sistare@oracle.com>]

On 4/9/2025 9:34 AM, Markus Armbruster wrote:
> Steven Sistare <steven.sistare@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@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.

- Steve