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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Bob Liu [mailto:bob.liu@xxxxxxxxxx]
> Sent: 16 March 2016 13:59
> To: Ian Jackson
> Cc: xen-devel@xxxxxxxxxxxxx; Paul Durrant; konrad.wilk@xxxxxxxxxx;
> jgross@xxxxxxxx; Roger Pau Monne; annie.li@xxxxxxxxxx
> Subject: Re: [RFC PATCH] blkif.h: document scsi/0x12/0x83 node
> 
> 
> On 03/16/2016 08:36 PM, Ian Jackson wrote:
> > Bob Liu writes ("[RFC PATCH] blkif.h: document scsi/0x12/0x83 node"):
> >> Sometimes, we need to query VPD page=0x83 data from underlying
> >> storage so that vendor supplied software can run inside the VM and
> >> believe it's talking to the vendor's own storage.  But different
> >> vendors may have different special features, so it's not suitable to
> >> export through "feature-xxxx".
> >>
> >> One solution is query the whole VPD page through Xenstore node, which
> has
> >> already been used by windows pv driver.
> >>
> http://xenbits.xen.org/gitweb/?p=pvdrivers/win/xenvbd.git;a=blob;f=src/x
> envbd/pdoinquiry.c
> >
> > Thanks for your contribution.
> >
> > Thanks also to Konrad for decoding the numbers, which really helps me
> > understand what is going on here and helped me find the relevant
> > references.
> >
> > (For background: I have just double-checked the SCSI spec and: INQUIRY
> > lets you query either the standard page, or one of a number of `vital
> > product data' pages, each identified by an 8-bit page number.  The VPD
> > pages are mostly full of vendor-specific data in vendor-specific
> > format.)
> >
> > I have some qualms about the approach you have adopted.  It is
> > difficult to see how this feature could be used safely without
> > knowledge specific to the storage vendor.
> >
> > But I think it is probably OK to define a specification along these
> > lines provided that it is very clear that if you aren't the storage
> > vendor and you use this and something breaks, you get to keep all the
> > pieces.
> >
> >> + * scsi/0x12/0x83
> >> + *        Values: string
> >> + *        A base64 formatted string providing VPD pages read out from
> backend
> >> + *        device.
> >
> > I think this probably isn't the prettiest name for this node or
> > necessarily the best format but given that this protocol is already
> > deployed, and this syntax will do, I don't want to quibble.
> >
> > I would like the base64 encoding to specified much more explicitly.
> > Just `base64 formatted' is too vague.
> >
> >
> >> + *        The backend driver or the toolstack should write this node with 
> >> VPD
> >> + *        informations when attaching devices.
> >
> > I think this is the wrong semantics.  I certainly don't want to
> > encourage backends to use this feature.
> >
> > Rather, I would prefer something like this:
> >
> >  * scsi/0x12/0x<NN>
> >
> >    This optional node contains SCSI INQUIRY VPD information.
> >    <NN> is the hexadecimal representation of the VPD page code.
> >
> >    A frontend which represents a Xen VBD to its containing operating
> >    system as a (virtual) SCSI target may return the specified data in
> >    response to INQUIRY commands from its containing OS.
> >
> >    A frontend which supports this feature must return the backend-
> >    specified data for every INQUIRY command with the EVPD bit set.
> >    For EVPD=1 INQUIRY commands where the corresponding xenstore node
> >    does not exist, the frontend must report (to its containing OS) an
> >    appropriate failure condition.
> >
> >    A frontend which does not support this feature (ie, which does not
> >    use these xenstore nodes), and which presents as a SCSI target to
> >    its containing OS, should support and provide whatever VPD
> >    information it considers appropriate, and should disregard these
> >    xenstore nodes.
> >
> >    A frontend need not - and often will not - present to its
> >    containing OS as a device addressable with SCSI CDBs.  Such a
> >    frontend has no use for SCSI INQUIRY VPD information.
> >
> >    A backend should set this information with caution.  Pages
> >    containing device-vendor-specific information should not be
> >    specified without the appropriate device-vendor-specific knowledge.
> >
> 
> That's much more clear, thank you very much!
> 
> >
> > Also I have two other observations:
> >
> > Firstly, AFAICT you have not provided any way to set the standard
> > INQUIRY response.  Is it not necessary in your application to provide
> 
> If backends are not encouraged to use this node, then we must have the
> toolstack write this node with the right VPD information.
> Paul mentioned there should be corresponding code in the xapi project, but I
> haven't found out where.
> 
> 
> > synthetic vendorid and productid, at the very least ?
> >
> > Secondly, I think your hope that
> >
> >> blkfront in Linux ... can use the same mechanism.
> >
> > is I think misguided.  blkfront does not present the disk (to the rest
> > of the Linux storage system) as a SCSI device.  Rather, Linux allows
> > blkfront to present as a block device, directly, and this is what
> > blkfront does.
> >
> 
> But we'd like to get the VPD information(of underlying storage device) also in
> Linux blkfront, even blkfront is not a SCSI device.
> 
> That's because our underlying storage device has some vendor-specific
> features which can be recognized through informations in VPD pages.
> And Our applications in guest want to aware of these vendor-specific
> features.

I think the missing piece of the puzzle is how the applications get this 
information. In Windows, since everything is a SCSI LUN (or has to emulate one) 
applications just send down 'scsi pass-through' IOCTLs and get the raw INQUIRY 
data back. In Linux there would need to be some alternative scheme that 
presumably blkfront would have to support.

  Paul

> 
> Regards,
> Bob
> 


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