Re: [Qemu-devel] [PATCH v2 0/2] Add dbus-vmstate

[Marc-André Lureau]

Hi

On Fri, Aug 23, 2019 at 3:21 PM Daniel P. Berrangé <address@hidden> 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/address@hidden/address@hidden/

as well as the libvirt series
https://patchew.org/Libvirt/address@hidden/


Other approaches are common prefix (ex:
org.mpris.MediaPlayer2.FooBar), which also allows to identify a
particular implementation in a simple way.

>
> For things we want every vhost user daemon to implement, we can
> define well known objects paths & intefaces to expose at the path.
>
> eg If the helper supports vmstate dump, it should export an object
> at the well known path "/org/qemu/VMState" with methods x, y & z
>
> If the helper supports debug logging it should export an object at
> the well known path "/org/qemu/Logger" with method a, b & c
>
> etc
>
> Regards,
> Daniel
> --
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