Re: [Qemu-devel] KVM call minutes April 3

[Michael Roth]

On Wed, Apr 04, 2012 at 01:53:34PM +0300, Dor Laor wrote:
> On 04/04/2012 01:37 PM, Michael Roth wrote:
> >
> >On Apr 4, 2012 2:42 AM, "Paolo Bonzini" <address@hidden
> ><mailto:address@hidden>> wrote:
> > >
> > > Il 04/04/2012 03:18, Michael Roth ha scritto:
> > > > Attacking the IDL/schema side first is the more rationale approach.
> >From
> > > > there we can potentially generate ASN.1 BER/DER visitors for the
> >protocol
> > > > side, or potentially even just vmstate bindings as a start. I've
> >recently
> > > > started looking into the latter... it's completely feasible, the only
> > > > downside is it complicates the IDL due requiring support for a lot of
> > > > what are very much vmstate-specific items, but it should be possible to
> > > > do this in a manner where those annotations are self-contained and
> > > > ignorable if we opted to replace vmstate-style declarations.
> > >
> > > We can also keep the current vmstate descriptions, but access fields
> > > from the automatically-generated visitors instead of struct fields.
> > > This keeps the IDL simple.
> >
> >It may be worthwhile as an incremental step though, one nice thing about
> >automatically generated bindings is that with the QIDL Anthony
> >prototyped a while back we assume we serialize by default, so changes in
> >annotated structs automatically trigger changes in the generated
> >bindings unless you explicitly mark fields as immutable/derivable/etc,
> >which we can tie into the build or make check to automatically detect
> >and bring attention to changes in vmstate. This may be worth the effort
> >if we adopt the proposed 4 year migration support cycle for pc-1.0,
> >since that'll continue to rely on vmstate even after we move on to an
> >IDL and newer protocol.
> 
> Beyond ASL/IDL I like to be sure that we're not just translating one
> format to other representation but instead we introduce some new
> functionality like:
>  - Ability to negotiate the protocol version
>  - Bi-direction data exchange, the sender will send data as a function
>    of the target release
>    - Include the machine type too

I've been toying with the notion of having the target start up a QMP
limited server that the source talks to to orchestrate negotiation. We
could potentially even send the device state by taking our QIDL-generated
visitors and serializing state via QmpOutputVisitor. QMP can be made
aware of the format of the device state input by taking the intermediate
step of generating QAPI schemas via QIDL, and using the QAPI code
generators to generate the visitors rather than QIDL directly. This
would also address the protocol side: just use QMP rather than ASN.1..

It's not as compact, but device state is such a small amount of data
compared to memory/disk that I don't think it's worth optimizing that
aspect, though we could use compression at the protocol layer if we
were inclined. Anything more suited to an out-of-band protocol, like
memory/disk, could be orchestrated via this interface... source can ask
target for a port to handle such things, negotiate stuff like XBZRLE,
etc.

>  - Synchronize the entire qemu cmdline and don't relay on management to
>    set it up.
>    - Along the way, deal w/ hotplug events.

My initial plan for the QIDL-generated visitors is to associate a QOM
property, "state", with each device, and to serialize device state by
walking the QOM composition tree, the main rationale being that if we
extend that serialization to include other QOM properties, I believe we
have everything we need to recreate all the devices on the target:
parent->child relationships, types, properties set via cmdline,
device state...

A simpler alternative would be to leverage just send the cmdline
options over to the target and assume that it results in the same underlying
machine, then just send off the device state. Much simpler actually...but
the above approach should work regardless of changes to the command-line
options on the source... having an internally stable cmdline scheme
might work as well...

I'm not sure what the right approach is here but whatever we decide on I think
being able to automatically generate visitors from annotations is a good
first-step and should tie into any forseeable approaches.

> 
> >
> > >
> > > Paolo
> > >
> >
>