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

[Michael S. Tsirkin]

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" <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's wrong with adding a disable-modern flag, like pci has?
User can set that to get a legacy device.

> 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.

-- 
MST