Re: [RFC 0/6] vhost/scsi: Add T10 PI SGL passthrough support

Nicholas A. Bellinger
2014-02-25T06:05:11Z

On Mon, 2014-02-24 at 11:23 +0100, Paolo Bonzini wrote:
> Il 24/02/2014 06:32, Nicholas A. Bellinger ha scritto:
> > AFAICT up until this point the ->prio field has been unused, but
> > I'm certainly open to better ways of signaling (to vhost) that some
> > number of metadata iovs are to be expected.. Any thoughts..?
>
> Hi nab,
>
> the virtio-scsi side of the patch is nice and readable. As requested,
> here are my thoughts on how to add it to the standard.
>
> The ->prio field is there to mimic SAM's command priority field (8.7 in
> my copy of the standard). I'd rather leave it alone; I understand this
> is the main reason why this patch is RFC.

Yes. ;)

>
> Since we have a new feature bit, we can add a new element before the
> cdb. It could be a count of scatter/gather list like you did here, or
> it could be a byte count. Even better, we can add _two_ new fields, one
> for protection data out and one for protection data in.
>

Having two 16-bit fields for data out / data in protection count in the
command header should be fine.

So that said, adding a new virtio_scsi_cmd_req_pi definition per your
recommendation, and will update the series to use this when the
VIRTIO_SCSI_F_T10_PI feature bit has been negotiated on both ends.

> Also, do we need an equivalent of the residual field, but for metadata?
>

Mmm, at least for PI I don't think a residual field is necessary.

Any time the metadata is not fully read on outgoing WRITEs, or written
on incoming READs the next hop performing a VERIFY operation will end up
failing with a GUARD or REFERENCE TAG failure.

MKP..?

> Finally, any reason why you put the data sg elements before the metadata
> sg elements?

Nope, no particular reason for this.

> I would have thought that processing is a bit simpler if
> either the metadata comes first, or you store in the command header the
> data count (either sg or byte). Because the virtio buffers form a
> linked list, it's a bit backwards to put metadata last, and store
> metadata count in the command header; it prevents you from processing
> the buffers online because you don't know when the metadata starts.
> Even though the Linux virtio layer always gives you a buffer count, this
> need not be the case in general.
>

No objection here. Updating the patch series to place protection
information ahead of the actual data payload.

--nab