Re: [Qemu-devel] [RFC] Migration capability negotation

[Peter Lieven]

On 25.10.2013 07:42, Eric Blake wrote:
> On 10/25/2013 04:27 AM, Peter Lieven wrote:
>
> > Ok, one way direction - i forgot about this paradigm.
> >
> > 2 thoughts:
> >
> > a) a send-capabilities capability that "stores" the capabilities that where
> > used when savevm was used. I would implement a special segment
> > right at the beginning of the data stream that has all capabilities listed that
> > where set and that ultimately must be supported to import a saved state
> > under any circumstances. capabilities that are only have a meaning at
> > the source VM should not be set. if there is an unsupported capability
> > set the import can be aborted right at the beginning.
> Sounds reasonable; but ideally, it would either have to be in such a way
> that doesn't break back-compat with older qemu, or else you have
> invented a new file format; and once you invent a new file format, we
> might as well make the file format sane by being FULLY self-describing
> (see also Alexander Graf's work from KVM forum on adding a migrate-debug
> device).  That is, don't just require the same capabilities, but also
> require all other command line arguments to be sane in comparison to
> what the savevm image was using.
what I had in mind when I thought about this was my old idea to skip
zero pages in ram migration. it turned out that there is a problem because
the target vm memory is not clean as expected but contains things like roms,
bios etc so we had to remove this.

when there is a way by ensuring that the target vm memory is clean either
by avoiding to load data into the ram or the hacky way to erase contents at
beginning of ram_load it must be somewhere in the datastream that the
zero blocks where left out. i thought to have this either by a capability
info block or by a special ram block with a flag that means erase all memory.
the latter would ensure that the memory is clean regardless if the capability
is set in the destination and secondly that the migration load fails if the
flag is unknown.

> > b) an extension the the qmp-migrate-capabilities or a new command that give
> > the controlling process (e.g. libvirt) a hint which features are a good thing 
> > to turn on
> > if they are supported on both sides (e.g. zero-blocks in block-migration).
> Not really needed.  New capabilities must be off by default (back-compat
> reasons), so the only time they will be turned on at the source is if
> the management (such as libvirt) is smart enough to know what the
> capability does; once you can assume that, you can also assume the
> management knows how to set up the destination properly.  Which is why
> what we have already works (making management do all the negotiation
> correctly).  Yes, maybe qemu could make it easier or more foolproof, but
> since management already has to handle the job (particularly because
> management might be dealing with older qemu that doesn't have the
> ease-of-use additions), I'm not sure the effort of extra code in qemu is
> worth the effort.
I understand. I am ok with saying its the management apps task
to negotiate capabilities.

Peter

Peter