[libvirt] Fwd: [PATCH] qemu: Add cmd_per_lun, max_sectors to virtio-scsi
Nicholas A. Bellinger
nab at linux-iscsi.org
Thu Jul 3 02:08:35 UTC 2014
On Wed, 2014-07-02 at 18:48 -0600, Eric Blake wrote:
> On 07/02/2014 06:28 PM, Nicholas A. Bellinger wrote:
>
> >> QEMU is not the only hypervisor that libvirt targets, so tieing libvirt
> >> names to QEMU names is a non-goal. We pick the names that make most sense
> >> in the context of libvirt.
> >>
> >
> > Not sure I follow.. virtio-scsi is specific to QEMU/KVM, and per the
> > comment in the original patch:
> >
> > 'Currently it only supports attribute <code>queues</code> (<span class="since">1.0.5</span>, QEMU and KVM only)'
> >
> > would seem to indicate the parameter names are only used in the context
> > of QEMU/KVM, no..?
>
> Just because qemu is the only hypervisor driver that _currently_ uses
> the feature doesn't preclude the libxl hypervisor from _also_ gaining
> support for the feature in a future libvirt release, at which point the
> documentation would mention the new version number for the additional
> use of the feature. Again, the name qemu chose is not necessarily the
> best name compared to what it might map to in libxl or any other
> hypervisor, so libvirt tries to pick names that are consistent with
> other libvirt terms, even if they don't match underlying qemu names.
>
It's worth mentioning that these names are coming directly from the
Linux SCSI subsystem tunables that are read from virtio-scsi config
space and set during drivers/scsi/virtio_scsi.c:virtscsi_probe() init
time, and did *not* originate with QEMU.
> >
> > If the virtio-scsi parameters are intended to be used across
> > hypervisors, then matching them to QEMU's own names doesn't really
> > matter. But if they are specific to virtio-scsi and only used by
> > QEMU/KVM instances, then renaming them to something arbitrary to libvirt
> > is pointless and confusing.
>
> virtio is not necessarily a qemu-only concept.
>
The point is that if a potential port of virtio-scsi to Xen ever
happens, it will use the same parameter names that every Linux SCSI LLD
has been using for the last 20 years. Eg: They will be the same
parameters names that the virtio-scsi LLD pulls out of configuration
space, regardless of if it's KVM or XEN.
--nab
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