Hi On Fri, Aug 23, 2019 at 3:42 PM Daniel P. Berrangé <berra...@redhat.com> wrote: > > On Fri, Aug 23, 2019 at 03:31:16PM +0400, Marc-André Lureau wrote: > > Hi > > > > On Fri, Aug 23, 2019 at 3:21 PM Daniel P. Berrangé <berra...@redhat.com> > > wrote: > > > > > > On Thu, Aug 08, 2019 at 07:03:23PM +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 > > > > peer address, and save/load from org.qemu.VMState1 interface. > > > > > > > > This way, helpers can have their state migrated with QEMU, without > > > > implementing another 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) > > > > > > > > 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. > > > > > > So yesterday Stefanha brought up need for "mgmt apis" on the > > > virtiofsd helper process & the conclusion is that dbus makes > > > most sense for this purpose: > > > > > > https://www.redhat.com/archives/virtio-fs/2019-August/msg00339.html > > > > > > This use case is a slightly different from vmstate though. > > > > > > For vmstate we have two parties - virtiofsd and QEMU talking > > > > > > For the "mgmt apis" in virtiofsd, we have arbitrary parties > > > involved - virtiofsd *and* an admin client tool, and/or > > > maybe libvirt. > > > > > > I think this different scenario means that we do in fact need > > > to have a bus present, as the p2p model doesn't scale well > > > to many clients. > > > > > > Even if we have 1 dbus-daemon per QEMU instance, we need to cope > > > with multiple instances of the same helper needing to connect. > > > So we need to come up with some for identifying services. Normally > > > DBus only allows 1 peer to own a given well known service name at > > > any time. So we can't simply talk to a well-known 'org.qemu.virtiofsd' > > > service name. > > > > > > Each service would need to to just rely on exporting objects under > > > its unique service id (they look like :1.NNNN for some uniq NNN) > > > > > > QEMU still needs to known which connections on the bus are actually > > > providing vhost-user services, and which are other things (like > > > libvirt or random mgmt tools) > > > > > > So perhaps QEMU should expose a service 'org.qemu.VhostUserManager' > > > with an object /org/qemu/VhostUSerManager > > > > > > Each helper supporting vmstate could register its existance > > > by invoking a method > > > > > > org.qemu.VhostUserManager.Register(":1.NNNN") > > > > > > There is no need for extra registration if the services are queued. > > You can then query the queue of org.qemu.VhostUser instances. > > > > This is the approach I took in v1 of this series with > > org.qemu.VMState1 service name. > > > > See > > https://patchew.org/QEMU/20190708072437.3339-1-marcandre.lur...@redhat.com/20190708072437.3339-4-marcandre.lur...@redhat.com/ > > I think that's a pretty gross hack tha is abusing the unique service > concept, as we clearly don't have unique services anymore.
"well-known" names are not "unique". I think you are restricting what "well-known" names are and how to use them. The queued owner concept has always been there. > > > Other approaches are common prefix (ex: > > org.mpris.MediaPlayer2.FooBar), which also allows to identify a > > particular implementation in a simple way. > > This means QEMU still has to iterate over every single client > on the bus to identify them. If you're doing that, there's > no point in owning a well known service at all. Just iterate > over the unique bus names and look for the exported object > path /org/qemu/VMState Not exactly, since it wouldn't have to query each connection, but only the bus. > > > 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 :|
