On Wed, 15 Jul 2015 13:59:00 +0300 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Tue, Jul 14, 2015 at 07:43:44PM +0200, Cornelia Huck wrote: > > > Yes, and that's because as written, transitional devices must set > > > ANY_LAYOUT, but that's incompatible with scsi. > > > > Hm, I had a patch before that dynamically allowed different feature > > sets for legacy or modern, not only a subset. Probably won't apply > > anymore, but I'd like to able to do the following: > > > > - driver reads features without negotiating a revision: driver is > > legacy, offer legacy bits > > - driver negotiates revision 0: dito > > - driver negotiates revision >= 1: driver is modern, offer modern bits > > > > That way we could offer SCSI and !ANY_LAYOUT (if scsi is enabled) in the > > first two cases, and a new qemu could still offer scsi to old guests. > > > > Would it be worth pursuing that idea? > > Frankly, I don't think so: I don't see why it makes sense > to expose more features on the legacy interface than > on the modern one. Imagine updating drivers to fix a bug > and losing some features. How does this make sense? I don't think one should be a strict subset of the other. But I think we don't want to withdraw features from legacy guests on qemu updates either? > > I think the virtio TC's assumption was that the scsi passthrough was a > bad idea, so in QEMU we only keep it around for legacy devices to avoid > regressions. I'm not opposing this :) > > If you disagree and think transitional devices need the SCSI feature, > either try to convince pbonzini or rewrite the spec youself > to support it in the virtio 1 mode. This seems to boil down to the different meaning of "transitional" for ccw and pci, see the other thread.
