On Mon, Jul 08, 2019 at 11:24:34AM +0400, Marc-André Lureau wrote: > 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) > > I chose D-Bus as it 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) > > The D-Bus protocol can be made to work peer-to-peer, but the most > common and practical way is through a bus daemon. This also has the > advantage of increased debuggability (you can eavesdrop on the bus and > introspect it).
The downside of using the bus daemon is that we have to spawn a new instance of dbus-daemon for every QEMU VM that's running on the host, which is yet more memory overhead for each VM & another process to manage, and yet another thing to go wrong. QEMU already has a direct UNIX socket connection to the helper processes in question. I'd much rather we just had another direct UNIX socket connection to that helper, using D-Bus peer-to-peer. The benefit of debugging doesn't feel compelling enough to justify running an extra daemon for each VM. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
