Re: [Qemu-devel] [PATCH 3/3] virtio-blk: note optional features

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/06/2012 11:12 AM, Paolo Bonzini wrote:
> Il 06/03/2012 18:03, Anthony Liguori ha scritto:
>> I don't know how comfortable I feel about this.
>>
>> You can't just remove a feature in flight.  The guest is going to behave
>> differently in such a way that the host isn't expecting.  Yes, it should
>> fail gracefully, but nonetheless it will fail.
>>
>> Aren't you just delaying the inevitable?  Instead of having migration
>> fail, the guest workload is going to fail.  How is this an improvement?
>
> VIRTIO_BLK_F_SCSI feature was almost never used but was always marked as
> available.  Because of possible security problems connected to it,
> libvirt started making it an opt-in feature.
>
> In practice, you need to configure your host specially if you want to
> use SCSI passthrough (e.g. you must not use labels and UUIDs in your
> /etc/fstab), so it's safe to assume that guests that have SG_IO disabled
> under their feet will keep working.
>
> That said, instead of this hack we can just decouple scsi=on/off from
> VIRTIO_BLK_F_SCSI, and just report the feature.  After all we do not
> clear VIRTIO_BLK_F_SCSI just because the device is backed by a file or
> partition, yet SG_IO is still unavailable in those cases.  I'll send
> patches for this tomorrow.

Okay, that makes more sense to me.  I think this is an exceptional circumstance 
so handling it as a one-off verses a generic mechanism would be preferred.

Regards,

Anthony Liguori

>
> Paolo