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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 22, 2015 at 12:55:22PM +0200, Cornelia Huck wrote:
> On Wed, 22 Jul 2015 13:44:14 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Jul 22, 2015 at 12:38:40PM +0200, Cornelia Huck wrote:
> > > 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.
> > 
> > Paolo's argument is that no one should be using scsi passthrough.
> > 
> > If the feature has users, we should bring it back into virtio 1.
> > 
> > If almost one uses it, then no one will suffer too much from getting
> > an error message saying "please set disable-modern=on".
> 
> And here's where we disagree. Even if it's exotic, I don't want to
> break existing users.

You should take this disagreement to the virtio TC.  QEMU merely
implements what the spec voted by TC says.

> > And there's no reason to make it behave differently
> > between ccw and pci.
> > 
> > 
> > > > 
> > > > 
> > > > > 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.
> > 
> > You likely can just add these unconditionally.
> 
> Backwards compatibility?

Compatibility is built-in to revision negotiation, isn't it?

> > How is this related to virtio blk at all?
> 
> It isn't, revision handling is generic. It's just the scsi bit that
> triggered it.

-- 
MST