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

[Michael S. Tsirkin]

On Wed, Jul 15, 2015 at 11:11:18AM +0200, 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.

I think the issue is theoretical since linux guests
do not read feature bits > 31 unless revision 1
is negotiated.

In any case, won't the following solve it nicely?

if (revision >= 1)
                return d->host_features
else
                return d->host_features & 0xffffffff;


-- 
MST