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

[Markus Armbruster <armbru@redhat.com>]

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?

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