[This is an RFC series, as being marked out. It is trying to collect
opinions. It's not for merging yet]
Background
==========
It all starts with machine compat properties..
Machine compat properties are the major weapon we use currently in QEMU to
define a proper guest ABI, so that whenever we migration a VM instance from
whatever QEMU version1 to another QEMU version2, as long as the machine
type is the same, logically the ABI is guaranteed, and migration should
succeed. If it didn't, it's a bug.
These compat properties are only attached to qdev for now. It almost
worked.
Said that, it's also not true - we already have non-qdev users of such, by
explicitly code it up to apply the compat fields. Please refer to the
first patch commit message for details (meanwhile latter patches will
convert them into a generic model).
Obviously, we have demands to leverage machine compat properties even
outside of qdev. It can be a network backend, it can be an object (for
example, memory backends), it can be a migration object, and more.
This series tries to introduce a common root class OBJECT_COMPAT for it. I
didn't abuse OBJECT because I know there're too many OBJECTs that do not
need compat properties at all. With this design, we can also opt-in piece
by piece on the new root class, only when needed.
Class OBJECT_COMPAT
===================
This is almost OBJECT class, except that it'll also apply machine compat
properties from anywhere. One can refer to patch 1.
Note that currently I didn't further identify the three possible source of
object_compat_props[3] (accel, machine compat property, legacy globals). I
don't think it's a huge issue so far because non-qdev objects will not
collapse with names in accel / legacy globals, due to the fact that object
names cannot dup acorss QEMU binary. So I kept the changeset as minimum as
possible. Feel free to shoot if there's concerns I overlooked.
This part is done in patch 1-6. This is the part I felt slightly more
confident with. Meanwhile, these will be the dependency if we want to
e.g. allow TAP network backends to take compat properties like a virtio-net
frontend (but likely we'll need to QOMify TAP first). That's something for
the future even if applicable.
Export Property from QDEV
=========================
I also have patch 7-10 below for one step further beyond OBJECT_COMPAT.
Feel free to take it even as a seperate small series to review.
So far the first part will be the focus, but I still want to collect
opinions here on this second part. This is about exporting Property for
non-qdev uses. Currently, migration is the only user.
In short, Property is something qdev uses internally to ultimately
represents ObjectClass's properties hash table. It's pretty handy to
e.g. avoid definining accessors for object properties, setting default
values, etc. Then they'll be converted to Object properties at some point.
Migration object currently defines all the global fields in Property and
can use "-global migration.XXX" to allow global overrides, with almost one
line for each property, which is efficient.
This 2nd step will allow migration object to inherit from OBJECT_COMPAT too
with almost only a few lines of changes, and keep the functionality as-is.
Two other options we have:
(1) Keep migration object to be a qdev, it's still fine, even if it
sounds hackish.. if we want to keep "-global" working as before
(2) Inherit OBJECT_COMPAT without supporting "-global" anymore.
Any comments welcomed, especially on the first half (1-6), thanks.
Peter Xu (10):
qom: Introduce object-compat
qdev: Inherit from TYPE_OBJECT_COMPAT
hostmem: Inherit from TYPE_OBJECT_COMPAT
accel: Inherit from TYPE_OBJECT_COMPAT
confidential guest support: Inherit from TYPE_OBJECT_COMPAT
qom: Unexport object_apply_compat_props()
qdev: Pave way for exporting Property to be used in non-qdev
qdev: Introduce helper object_apply_globals()
qdev: Refactor and rename of qdev_class_add_property()
migration: Inherit from TYPE_OBJECT_COMPAT
include/hw/qdev-properties.h | 11 ++++++++
include/qom/object.h | 2 +-
migration/migration.h | 2 +-
accel/accel-common.c | 2 +-
backends/confidential-guest-support.c | 2 +-
backends/hostmem.c | 8 +-----
hw/core/qdev-properties.c | 37 +++++++++++++++++++++------
hw/core/qdev.c | 6 ++---
migration/migration.c | 31 +++++++++++-----------
qom/object.c | 16 +++++++++++-
system/vl.c | 1 -
target/i386/sev.c | 1 -
12 files changed, 79 insertions(+), 40 deletions(-)
--
2.50.1
