Re: [Qemu-devel] [RFC PATCH] implement vmware pvscsi device

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Apr 18, 2011 at 3:05 PM, Hannes Reinecke <hare@suse.de> wrote:
> On 04/15/2011 10:56 PM, Paolo Bonzini wrote:
>>
>> On 04/15/2011 05:04 PM, Stefan Hajnoczi wrote:
>>>
>>> The way I approached virtio-scsi was to look at the SCSI Architecture
>>> Model document and some of the Linux SCSI code. I'm not sure if
>>> letting virtio-blk SCSI pass-through or scsi-generic guide us is a
>>> good approach.
>>>
>>> How do your ioprio and barrier relate to SCSI?
>>
>> Both are part of the transport protocol, which can provide
>> additional features with respect to SAM. For example SCSI doesn't
>> provide the full details of hotplug/hotunplug, or doesn't have a way
>> for the guest to trigger a drive unplug on the host, but these are
>> all desirable features for virtio-scsi (and they are supported by
>> vmw_pvscsi by the way).
>>
> And this is something I really miss in the current proposals, namely
> a working transport layer.
>
> The SCSI spec (SPC etc) itself just handles command delivery between
> initiator and target. Anything else (like hotplug, error recovery, target
> addressing etc) is out of the scope of the spec and needs to be implemented
> on another layer (that's the ominous
> transport layer).
>
> Hence any protocol implemented to the above spec would be missing those
> parts, and they would need to be implemented additionally.
> Which also explains why these features are missing when just using SCSI CDBs
> as the main command container.
>
> My proposal would be to implement a full virtio-scsi _host_, and extend the
> proposal to be able to handle the transport layer too.
> At the lastest we would need to include a LUN address before the CDB, and
> define TMF command values for proper error recovery.
>
> That way we could handle hotplug / -unplug via a simple host rescan, and
> would even be able to pass-in NPIV hosts.

In my prototype there is a header and a footer for the request and
response, respectively:
http://git.kernel.org/?p=linux/kernel/git/nab/lio-core-2.6.git;a=blob;f=include/linux/virtio_scsi.h;hb=refs/heads/tcm_vhost

We definitely need more than plain CDB pass-through.

Stefan