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

[Michael S. Tsirkin]

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.

-- 
MST