Re: missing unplug of SCSI devices in HVM guest

[George Dunlap <dunlapg@umich.edu>]

On Wed, Aug 24, 2016 at 10:24 AM, Olaf Hering <olaf@aepfle.de> wrote:
> Does anyone remember why the the vbd frontend drivers also claim the
> SCSI disks, but the vbd backend in qemu has no unplug support for SCSI?
>
> The current situation for qemu-xen and qemu-xen-traditional is that both
> will create an emulated LSI controller with disk=[vdev=sda]. The
> xenlinux and pvops frontend drivers will claim the disk, but the
> sym53c8xx will see and claim it as well. As a result each disk is
> visible twice. One has to blacklist the sym53c8xx driver to avoid that.
>
> What should be done to fix this?
> #1 new unplug protocol for SCSI, but old guests dont know about it
> #2 reuse IDE flag to also unplug SCSI. This would cover pvops and guests
>    where xenlinux based xen-platform-pci.ko is loaded before
>    sym53c8xx.ko. It would break guests with frontend drivers that do not
>    claim SCSI disks, the SCSI disk would disappear (if such frontends
>    really exist).
> #3 do not provide SCSI if guest has PV drivers

I think #3 was has been done in practice, but obviously not enforced
by the toolstack -- i.e., "Doctor, it hurts when I do this <makes
motion>."  "Then don't do that."

The problem with enforcing #3 is that there's no real way for the
toolstack to know if the guest will have PV drivers before booting.

There's also #4: Do not provide a PV backend for SCSI disks.  Not sure
that's actually possible, as libxl has historically used the PV
backend as the canonical place for listing disks associated with a
domain (although that may have changed since XSA-whatever which
resulted in libxl having its own local copy of the backend
information).

Paul, do you have any thoughts?

 -George

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