Re: [Qemu-devel] [PATCH V2 0/5] Set correct blk feature for virtio 1.0

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

On Wed, Jul 15, 2015 at 01:38:42PM +0200, Cornelia Huck wrote:
> On Wed, 15 Jul 2015 17:39:53 +0800
> Jason Wang <jasowang@redhat.com> wrote:
> 
> > 
> > 
> > On 07/15/2015 05:11 PM, Cornelia Huck wrote:
> > > On Wed, 15 Jul 2015 13:29:56 +0800
> > > Jason Wang <jasowang@redhat.com> wrote:
> > >
> > >> Hi all:
> > >>
> > >> This series tries to set feature correctly for virtio-blk when virtio
> > >> 1.0 is supported. Two isssues were addressed according to the spec:
> > >>
> > >> - scsi passthrough was not support in 1.0. This is done by, 1) disable
> > >>   scsi by defautl for 2.4 machine type and fail the initialization
> > >>   when both scsi and 1.0 were set.
> > >> - any layout must be set for transitional device. This is done by set
> > >>   any layout when 1.0 is supported.
> > >>
> > >> Please review
> > >>
> > >> Changes from V1:
> > >> - Split virtio-net changes out of the series
> > >> - Enable VIRTIO_BLK_F_SCSI only when scsi is set
> > >> - Disable scsi by default and compat it for legacy machine types
> > >> - Let get_features() can fail and fail the initialization of
> > >>   virito-blk when both 1.0 and scsi were supported.
> > > Hm, this seems confusing to me mainly due to the different way
> > > transitional devices are handled by pci and ccw.
> > >
> > > For virtio-pci: (please correct me if I misunderstood)
> > > - devices (except input) are transitional by default
> > > - user can disable legacy or modern support
> > > - drivers can use method they prefer, depending on VERSION_1
> > >
> > > For virtio-ccw:
> > > - transitional means "in limbo" regarding legacy or modern
> > > - devices become legacy if features are read without negotiation of a
> > >   revision, or if revision 0 is negotiated
> > > - they become modern if revision 1 is negotiated
> > >
> > > That implies that for ccw, a transitional device does not offer any
> > > features: It either transitions to the legacy or modern state if a
> > > revision was negotiated, or it becomes a legacy device by reading
> > > features (which are the legacy features, then). While pci has "real"
> > > transitional devices needing to offer a certain feature set.
> > >
> > > I'm not sure what the solution is here: basically, ccw needs a dynamic
> > > feature set, while pci does not.
> > >
> > 
> > So, if I understand correctly. All virtio-ccw devices are transitional
> > since user can disable neither legacy nor modern. It looks to me then we
> > can set VIRTIO_F_VERSION_1 unconditionally for ccw. Then there's no
> > issue left?
> 
> I think the base problem is that "transitional" for pci means something
> slightly different than "transitional" for ccw - it's more transient
> for ccw.
> 
> Setting VERSION_1 is not really the problem (as legacy guests will only
> retrieve the first 32 bits of features anyway), but any decicions about
> other feature bits that are made based upon the presence of VERSION_1.

Are we talking about the scsi bit here, again?
Or are there other examples?

-- 
MST