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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo