Re: [Xen-devel] [RFC PATCH] blkif.h: document scsi/0x12/0x83 node

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Jackson [mailto:Ian.Jackson@xxxxxxxxxxxxx]
> Sent: 22 March 2016 16:15
> To: Paul Durrant
> Cc: David Vrabel; Konrad Rzeszutek Wilk; Bob Liu; jgross@xxxxxxxx; xen-
> devel@xxxxxxxxxxxxx; annie.li@xxxxxxxxxx; Roger Pau Monne
> Subject: RE: [Xen-devel] [RFC PATCH] blkif.h: document scsi/0x12/0x83 node
> 
> Paul Durrant writes ("RE: [Xen-devel] [RFC PATCH] blkif.h: document
> scsi/0x12/0x83 node"):
> > It's getting hard to parse the thread at this point but, as I've
> > mentioned in a previous response in the thread, Windows basically
> > assumes disks are SCSI and it's up to the controller driver to make
> > it look that way.
> 
> Right.  That makes perfect sense.
> 
> > To that end the XENVBD Windows PV driver synthesizes VPD pages 80
> > and 83 but also have the ability pull base64 encoded VPD data from
> > xenstore. The synthesis is required to make Windows work properly
> 
> Right.
> 
> > but the reason for overriding it with data from xenstore is not
> > apparent.
> 
> Thanks, this is the part that we are struggling with and which it is
> proposed to now document.
> 
> > In XenServer the storage backend code does populate the
> > VPD information (with UUID information for the storage volume
> > created by XAPI) but I don't believe that this is necessary
> > behaviour in the normal case.
> 
> So the VPD here is a UUID from Xapi ?  Not anything do with the
> underlying storage ?
> 
> I think that is a plausible thing for Xapi to do.  Although I don't
> fully understand why it would be necessary, it doesn't seem
> implausible that there are guest OS's which look for and do something
> useful with such data (because they could use it, for example, to
> reliably identify a storage volume other than by reading its
> contents).
> 

I think that in the general case the VPD data is created by XAPI (or some 
element of the storage code) from the UUID. My belief is, though, there were 
cases where XAPI instead got the VPD data from the underlying storage.

> > My *assumption* is that, at some point
> > in the past, XenServer had OEM specific storage backends and the
> > requirement to run OEM s/w in guest which relied on VPD supplied by
> > the storage, and as is commonly the case xenstore was the easiest
> > way to get this information from the backend and into the guest.
> 
> Right, but the VPD information would be Xapi-generated and generic,
> rather than containing OEM information from the disk ?
> 

Not in the case of these (mythical) OEM backends.

I don't know whether it meets Oracle's use-case but I'm happy to add code in 
XENVBD to synthesize page 80 from a straightforward serial number in xenstore.

  Paul

> Ian.

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