Re: [Qemu-block] [Qemu-devel] [PATCH 2/5] virtio-blk: disable scsi passthrough for 1.0 device

[Cornelia Huck]

On Wed, 15 Jul 2015 17:11:57 +0300
"Michael S. Tsirkin" <address@hidden> 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 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.