Hi, With external processes or helpers participating to the VM support, it becomes necessary to handle their migration. Various options exist to transfer their state: 1) as the VM memory, RAM or devices (we could say that's how vhost-user devices can be handled today, they are expected to restore from ring state) 2) other "vmstate" (as with TPM emulator state blobs) 3) left to be handled by management layer
1) is not practical, since an external processes may legitimatelly need arbitrary state date to back a device or a service, or may not even have an associated device. 2) needs ad-hoc code for each helper, but is simple and working 3) is complicated for management layer, QEMU has the migration timing The proposed "dbus-vmstate" object will connect to a given D-Bus address, and save/load from org.qemu.VMState1 owners on migration. Thus helpers can easily have their state migrated with QEMU, without implementing ad-hoc support (such as done for TPM emulation) D-Bus is ubiquitous on Linux (it is systemd IPC), and can be made to work on various other OSes. There are several implementations and good bindings for various languages. (the tests/dbus-vmstate-test.c is a good example of how simple the implementation of services can be, even in C) dbus-vmstate is put into use by the libvirt series "[PATCH 00/23] Use a slirp helper process". v3: - after various discussions on helper processes, we settled on a preference for having a bus for communications. This version is actually v1 updated. - added a dbus.rst document to describe D-Bus recommendations for QEMU - added dbus-vmstate-daemon.sh to play with the dbus-daemon configuration (although it is not very useful in the context of a single UID) - added a new vmstate interface, so that any object can implement VMStateDescription, and converted dbus-vmstate - added "migration: fix vmdesc leak on vmstate_save() error" - convert to g_auto v2: - D-Bus is most common and practical through a bus, but it requires a daemon to be running. I argue that the benefits outweight the cost of running an extra daemon in v1 in the context of multi-process qemu, but it is also possible to connect in p2p mode as done in this new version. Marc-André Lureau (6): migration: fix vmdesc leak on vmstate_save() error vmstate: add qom interface to get id vmstate: replace DeviceState with VMStateIf tests: add qtest_expect_exit_status() docs: start a document to describe D-Bus usage Add dbus-vmstate object MAINTAINERS | 6 + backends/Makefile.objs | 4 + backends/dbus-vmstate.c | 530 ++++++++++++++++++++++++++++++++++ configure | 7 + docs/devel/migration.rst | 2 +- docs/interop/dbus-vmstate.rst | 68 +++++ docs/interop/dbus.rst | 73 +++++ docs/interop/index.rst | 2 + hw/block/onenand.c | 2 +- hw/core/Makefile.objs | 1 + hw/core/qdev.c | 21 +- hw/core/vmstate-if.c | 23 ++ hw/ide/cmd646.c | 2 +- hw/ide/isa.c | 2 +- hw/ide/piix.c | 2 +- hw/ide/via.c | 2 +- hw/misc/max111x.c | 2 +- hw/net/eepro100.c | 4 +- hw/net/vmxnet3.c | 2 +- hw/nvram/eeprom93xx.c | 4 +- hw/ppc/spapr_drc.c | 9 +- hw/ppc/spapr_iommu.c | 4 +- hw/s390x/s390-skeys.c | 2 +- include/hw/vmstate-if.h | 32 ++ include/migration/register.h | 6 +- include/migration/vmstate.h | 10 +- migration/qjson.h | 2 + migration/savevm.c | 29 +- stubs/vmstate.c | 4 +- tests/Makefile.include | 20 +- tests/dbus-vmstate-daemon.sh | 95 ++++++ tests/dbus-vmstate-test.c | 386 +++++++++++++++++++++++++ tests/dbus-vmstate1.xml | 12 + tests/libqtest.c | 41 +-- tests/libqtest.h | 9 + 35 files changed, 1355 insertions(+), 65 deletions(-) create mode 100644 backends/dbus-vmstate.c create mode 100644 docs/interop/dbus-vmstate.rst create mode 100644 docs/interop/dbus.rst create mode 100644 hw/core/vmstate-if.c create mode 100644 include/hw/vmstate-if.h create mode 100755 tests/dbus-vmstate-daemon.sh create mode 100644 tests/dbus-vmstate-test.c create mode 100644 tests/dbus-vmstate1.xml -- 2.23.0
