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

[Cornelia Huck]

On Wed, 15 Jul 2015 14:52:11 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Jul 15, 2015 at 01:38:42PM +0200, Cornelia Huck wrote:
> > On Wed, 15 Jul 2015 17:39:53 +0800
> > Jason Wang <address@hidden> wrote:
> > 
> > > 
> > > 
> > > On 07/15/2015 05:11 PM, Cornelia Huck wrote:
> > > > On Wed, 15 Jul 2015 13:29:56 +0800
> > > > Jason Wang <address@hidden> 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?
> 

The scsi bit is the one I thought about here; I'd need to check the spec
whether there are others (for other device types).