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.

To reduce this cost, consider QAPI calls that fetch more information in
each call:
  * qom-list-get-one: given a path, return a list of properties and values.
  * qom-list-get: 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.

In all cases, a returned property is represented by ObjectPropertyValue,
with fields name, type, and value.  If an error occurs when reading a value
the value field is omitted.  Thus an error for one property will not cause a
bulk fetch operation to fail.

To evaluate each method, I modified scripts/qmp/qom-tree to use the method,
verified all methods produce the same output, and timed each using:

  qemu-system-x86_64 -display none \
    -chardev socket,id=monitor0,path=/tmp/vm1.sock,server=on,wait=off \
    -mon monitor0,mode=control &

  time qom-tree -s /tmp/vm1.sock > /dev/null

I only measured once per method, but the variation is low after a warm up run.
The 'real - user - sys' column is a proxy for QEMU CPU time.

method               real(s)   user(s)   sys(s)  (real - user - sys)(s)
qom-list / qom-get   2.048     0.932     0.057   1.059
qom-list-get-one     0.402     0.230     0.029   0.143
qom-list-get         0.200     0.132     0.015   0.053
qom-tree-get         0.143     0.123     0.012   0.008

qom-tree-get is the fastest, reducing elapsed time by a factor of 14X,
and reducing QEMU CPU time by 132X.

qom-list-get is slower when fetching the entire tree, but can beat
qom-tree-get when only a subset of the tree needs to be fetched (not shown).
To keep things simple, provide only qom-list-get.

Changes in V4:
  * renamed qom-list-getv -> qom-list-get
  * expanded the qtest to verify a value
  * cosmetic changes in the docs
  * expanded the qom-list-get commit message

Changes in V3:
  * dropped qom-tree-get
  * modified the qom-tree script to use qom-list-getv
  * cosmetic changes in the docs and code.

Changes in V2:
  * removed "qom: qom_resolve_path", which was pulled separately
  * dropped the error member
  * fixed missing _list_tree in qom.py
  * updated 10.0 to 10.1

Steve Sistare (3):
  qom: qom-list-get
  python: use qom-list-get
  tests/qtest/qom-test: unit test for qom-list-getv

 python/qemu/utils/qom.py        |  43 ++++++++-------
 python/qemu/utils/qom_common.py |  53 ++++++++++++++++++
 qapi/qom.json                   |  50 +++++++++++++++++
 qom/qom-qmp-cmds.c              |  53 ++++++++++++++++++
 tests/qtest/qom-test.c          | 116 +++++++++++++++++++++++++++++++++++++++-
 5 files changed, 296 insertions(+), 19 deletions(-)

-- 
1.8.3.1

This feels ready.  Have a look at my (few) comments, and tell me what
you think.  Happy to apply finishing touches in my tree, without another
respin.

Thanks your your patience!

On 7/11/2025 11:06 AM, Markus Armbruster wrote:
> This feels ready.  Have a look at my (few) comments, and tell me what
> you think.  Happy to apply finishing touches in my tree, without another
> respin.
> 
> Thanks your your patience!

I'll send V5 with the final changes.

Thank you for your time Markus.  The result is more polished due to your
careful review.

- Steve