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

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed, 22 Jul 2015 13:32:17 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Jul 22, 2015 at 12:25:31PM +0200, Cornelia Huck wrote:
> > On Wed, 22 Jul 2015 12:21:32 +0300
> > "Michael S. Tsirkin" <mst@redhat.com> 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 <jasowang@redhat.com> 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 <jasowang@redhat.com>
> > > > > ---
> > > > >  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...
> 
> Having what work transparently? SCSI passthrough?
> Look, either you agree with Paolo or not.
> Paolo thinks it's a deprecated hack not really worth supporting long term.
> If you agree, I don't see why is asking for an extra property
> such a bit deal. If you don't agree - please open a new thread
> and argue about that.

I sometimes wonder whether we're arguing about the same thing :(

Dropping scsi for virtio-1 is fine. Dropping backwards-compatibility is
not. If I upgrade the host, I want the guests to be able to continue
using scsi without needing to fence virtio-1 off manually.

> 
> 
> > Moreover, I will need a revision-fencing mechanism in any case, when we
> > introduce further revisions.
> 
> Why? Assuming we drop more features in the future?

Revisions != features. Think new or changed channel commands, for
example.