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: 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.

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         0.402     0.230     0.029   0.143
qom-list-getv        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-getv 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-getv.

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-getv
  python: use qom-list-getv
  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          | 64 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 245 insertions(+), 18 deletions(-)

-- 
1.8.3.1