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

[Jason Wang <jasowang@redhat.com>]

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?