On Mon, 19 Aug 2013 16:05:43 +0300
"Michael S. Tsirkin" <> wrote:
> > One idea might be that all devices start out in 'legacy mode' and we
> > add either a feature bit or a new channel command that switches to
> > 'standard mode'. Old hosts would reject that command, while old guests
> > would simply not issue it.
> >
> > Another idea is to look at the read_conf/write_conf channel commands
> > and introduce a new pair of read_conf_le/write_conf_le commands. Again,
> > compatibility would be achived via the host doing command rejects.
> >
> > We also have the possibility to convey extra information about the
> > control unit (the device) in the 'extended sense id data' via the sense
> > id command. Basically, we can encode some supported channel commands in
> > there, but we would need a new command type for that architectured.
> > We'd still need to rely on the command rejects mentioned above, though.
>
> If guest can discover whether host supports the new layout,
> why would there be any rejects?
If we want to support old guests on new hosts, we'll have to rely on
this.
