On Wed, 15 Jul 2015 17:39:18 +0300 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Wed, Jul 15, 2015 at 04:30:51PM +0200, Cornelia Huck wrote: > > On Wed, 15 Jul 2015 17:11:57 +0300 > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > > > > > Fine, but revision is negotiated way before features are > > > > > > > probed so why does it make a practical difference? > > > > > > > > > > > > Legacy drivers (that don't know about the set-revision command) will > > > > > > read features without revision negotiation - we need to offer them > > > > > > the > > > > > > legacy feature set. > > > > > > > > > > Right. So simply do if (revision < 1) return features & 0xffffffff > > > > > and that will do this, will it not? > > > > > > > > Not for bits that we want to offer for legacy but not for modern. > > > > > > I don't think this selective offering works at least for scsi. > > > scsi is a backend feature, if you connect a modern device > > > in front the device simply does not work. > > > It therefore makes no sense to attach a transitional device > > > to such a backend. > > > > My point is that we're losing legacy features with that approach, and > > it would not be possible to offer them to legacy guests with newer > > qemus (at least with ccw). > > What's wrong with adding a disable-modern flag, like pci has? > User can set that to get a legacy device. The whole idea behind the revision-stuff was that we don't need something like disable-modern. If the device is able to figure out on its own if it is to act as a modern or a legacy device, why require user intervention? > > > What about the other way around (i.e. scsi is configured, therefore the > > device is legacy-only)? We'd only retain the scsi bit if it is actually > > wanted by the user's configuration. I would need to enforce a max > > revision of 0 for such a device in ccw, and pci could disable modern > > for it. > > Will have to think about it. > But I think a flag to disable/enable modern is useful in any case, > and it seems sufficient. I don't like the idea of disabling modern or legacy for ccw, where the differences between both are very minor. I also don't think requiring the user to specify a new flag on upgrade just to present the same features as before is a good idea: it is something that is easily missed and may lead to much headscratching.
