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

Steven Sistare <steven.sistare(a)oracle.com&gt; 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&gt; 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?