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

From: Ian Jackson <Ian.Jackson@eu.citrix.com>
To: Paul Durrant <Paul.Durrant@citrix.com>
Cc: "jgross@suse.com" <jgross@suse.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	"annie.li@oracle.com" <annie.li@oracle.com>,
	David Vrabel <david.vrabel@citrix.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [RFC PATCH] blkif.h: document scsi/0x12/0x83 node
Date: Tue, 22 Mar 2016 16:14:40 +0000	[thread overview]
Message-ID: <22257.28528.48906.729640@mariner.uk.xensource.com> (raw)
In-Reply-To: <f57d9c919c944ebcb317a34d208e08f5@AMSPEX02CL03.citrite.net>

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).

> 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 ?

Ian.

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