Re: [Qemu-devel] [PATCH V3 2/3] virtio-blk: fail get_features when both scsi and 1.0 were set

[Cornelia Huck]

On Wed, 22 Jul 2015 12:21:32 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Jul 22, 2015 at 10:58:43AM +0200, Cornelia Huck wrote:
> > On Wed, 22 Jul 2015 13:59:51 +0800
> > Jason Wang <address@hidden> wrote:
> > 
> > > SCSI passthrough was no longer supported in virtio 1.0, so this patch
> > > fail the get_features() when both 1.0 and scsi is set. And also only
> > > advertise VIRTIO_BLK_F_SCSI for legacy virtio-blk device.
> > > 
> > > Signed-off-by: Jason Wang <address@hidden>
> > > ---
> > >  hw/block/virtio-blk.c | 9 ++++++++-
> > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> > > index 4c27974..4716c3e 100644
> > > --- a/hw/block/virtio-blk.c
> > > +++ b/hw/block/virtio-blk.c
> > > @@ -731,7 +731,14 @@ static uint64_t virtio_blk_get_features(VirtIODevice 
> > > *vdev, uint64_t features,
> > >      virtio_add_feature(&features, VIRTIO_BLK_F_GEOMETRY);
> > >      virtio_add_feature(&features, VIRTIO_BLK_F_TOPOLOGY);
> > >      virtio_add_feature(&features, VIRTIO_BLK_F_BLK_SIZE);
> > > -    virtio_add_feature(&features, VIRTIO_BLK_F_SCSI);
> > > +    if (__virtio_has_feature(features, VIRTIO_F_VERSION_1)) {
> > > +        if (s->conf.scsi) {
> > > +            error_setg(errp, "Virtio 1.0 does not support scsi 
> > > passthrough!");
> > > +            return 0;
> > > +        }
> > > +    } else {
> > > +        virtio_add_feature(&features, VIRTIO_BLK_F_SCSI);
> > > +    }
> > > 
> > >      if (s->conf.config_wce) {
> > >          virtio_add_feature(&features, VIRTIO_BLK_F_CONFIG_WCE);
> > 
> > Do we advertise F_SCSI even if scsi is not configured in order to keep
> > the same bits as before? I'm afraid I don't remember, that thread was
> > long :/
> > 
> > I'm asking because I'd like to depend on that bit to decide whether I
> > can negotiate revision 1 for ccw and subsequently offer VERSION_1. It
> > would be an easy thing to do, and I'd like to avoid mucking around with
> > device-specific configuration from the transport.
> > 
> > To illustrate what I'm talking about, my current patchset for virtio-1
> > on ccw is here:
> > 
> > git://github.com/cohuck/qemu virtio-1-ccw-2.5
> 
> 
> I still think you are over-engineering it.
> 
> Just add a property to disable the modern interface.
> Anyone using scsi passthrough will have to set that,
> if not - above patch will make initialization fail.

And I still think requiring user action and not having this work
transparently is a bad idea...

Moreover, I will need a revision-fencing mechanism in any case, when we
introduce further revisions.