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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Wed, 22 Jul 2015 13:53:14 +0200
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Subject: Re: [Qemu-devel] [PATCH V3 2/3] virtio-blk: fail get_features when
 both scsi and 1.0 were set
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: cornelia.huck@de.ibm.com, libvir-list@redhat.com, Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>



On 22/07/2015 13:46, Daniel P. Berrange wrote:
> IIUC, the SCSI passthrough feature for virtio-blk is enabled by
> setting the 'scsi=on' property on the virtio-blk device, which is
> exposed by libvirt with XML:
> 
>     <disk type='block' device='lun'>
>       <driver name='qemu' type='raw'/>
>       <source dev='/dev/sda'/>
>       <target dev='vda' bus='virtio'/>
>     </disk>
> 
> (For use with virtio-scsi you'd just change the <target> element)
> 
> So if the guest is using virtio-1.0, then this will now fail to boot, or
> cause an error from monitor hotplug. This is not too bad, but I'm just
> wondering if there's anything else we ought to think about doing in libvirt
> in this situation. Normally we'd try to detect unsupported things upfront
> so we can report VIR_ERR_CONFIG_UNSUPPORTED, instead of the generic error
> code VIR_ERR_INTERNAL_ERROR, but perhaps this is sufficiently niche to
> not worry about it and its fine to just delegate error reporting to QEMU ?

Probably.  Note that it will be a long time before the default is
changed to 1.0 (if it ever will).  Perhaps you can start warning now
about <disk type='block' device='lun'>, and suggest using virtio-scsi
instead?

Paolo