Re: [Qemu-devel] [RFC PATCH v2 00/16] visitor+BER migration format

[Dr. David Alan Gilbert]

* Markus Armbruster (address@hidden) wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > On Tue, May 06, 2014 at 07:58:07PM +0100, Dr. David Alan Gilbert wrote:
> >> * Markus Armbruster (address@hidden) wrote:
> >> > "Michael S. Tsirkin" <address@hidden> writes:
> >> 
> >> <snip>
> >> 
> >> > > OK but for a new machine type, let's default to BER, right?
> >> > > I see no reason to keep supporting when non-BER when -M specifies 2.1
> >> > > compatibility, do you?
> >> > 
> >> > I fail to see the relation between machine type and migration's wire
> >> > encoding.
> >> 
> >> New machine types are a useful but not definitive line in the sand.  If
> >> you enable something/change the default on a new machine type you know
> >> it won't break any existing users since there aren't any.
> >> 
> >> Dve
> 
> The purpose of machine types is to keep the guest ABI stable.  I don't
> like tacking random crap unrelated to guest ABI to machine types.
> They're hard enough to grasp for users as they are.
> 
> > Exactly. And on the other hand, someone enabling old machine type
> > and doing live migration is likely to want to be compatible with old
> > qemu wrt migration.
> 
> Machine types let you migrate to a newer QEMU (forward migration)
> without messing up the guest ABI.  Migrating to an older QEMU (backward
> migration) basically doesn't work, and as long as that's the case,
> picking the older wire format by default is worthless.

Anyway, we seem to have had a long conversation about the least complicated
part of this patch set.

I'd love some thoughts on the actual visitor interface, which IMHO is the
bit that's actually messy and needs some rethinking).

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK