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

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: David Vrabel [mailto:david.vrabel@citrix.com]
> Sent: 22 March 2016 14:38
> To: Konrad Rzeszutek Wilk
> Cc: Bob Liu; Ian Jackson; jgross@suse.com; xen-devel@lists.xen.org;
> annie.li@oracle.com; Paul Durrant; Roger Pau Monne
> Subject: Re: [Xen-devel] [RFC PATCH] blkif.h: document scsi/0x12/0x83 node
> 
> On 22/03/16 14:10, Konrad Rzeszutek Wilk wrote:
> > On Tue, Mar 22, 2016 at 01:41:43PM +0000, David Vrabel wrote:
> >> On 22/03/16 12:55, Bob Liu wrote:
> >>>
> >>> On 03/17/2016 07:12 PM, Ian Jackson wrote:
> >>>> David Vrabel writes ("Re: [Xen-devel] [RFC PATCH] blkif.h: document
> scsi/0x12/0x83 node"):
> >>>>> On 16/03/16 13:59, Bob Liu wrote:
> >>>>>> But we'd like to get the VPD information(of underlying storage
> device) also in Linux blkfront, even blkfront is not a SCSI device.
> >>>>>
> >>>>> Why does blkback/blkfront need to involved here?  This is just some
> >>>>> xenstore keys that can be written by the toolstack and directly read
> by
> >>>>> the relevant application in the guest.
> >>>>
> >>>
> >>> They want a more generic way because the application may run on all
> kinds of environment including baremetal.
> >>> So they prefers to just call ioctl(SG_IO) against a storage device.
> >>>
> >>>> I'm getting rather a different picture here than at first.  Previously
> >>>> I thought you had some 3rd-party application, not under your control,
> >>>> which expected to see this VPD data.
> >>>>
> >>>> But now I think that you're saying the application is under your own
> >>>> control.  I don't understand why synthetic VPD data is the best way to
> >>>> give your application the information it needs.
> >>>>
> >>>> What is the application doing with this VPD data ?  I mean,
> >>>> which specific application functions, and how do they depend on the
> >>>> VPD data ?
> >>>>
> >>>
> >>> From the feedbacks I just got, they do *not* want the details to be in
> public.
> >>
> >> It is difficult to suggest how it should be done correctly without this
> >> information.
> >
> > Just think of it as a black box.
> 
> This isn't sufficient.
> 
> You are presenting a solution but have not properly described the
> problem so no one can evaluate whether the solution is appropriate.
> 
> >> I also find it difficult to see a use case where running the storage
> >> software in the guest (instead of in the backend) is sensible or desirable.
> >
> > Are you suggesting that doing backend drivers is not sensible?
> 
> I do not understand your question.
> 
> >>> Anyway, I think this is not a block of this patch.
> >>> In Windows PV block driver, we already use the same way to get the raw
> INQUIRY data.
> >>>  * The Windows PV block driver accepts ioctl(SG_IO).
> >>>  * Then it reads this /scsi/0x12/0x83 node.
> >>>  * Then return the raw INQURIY data back to ioctl.
> >>>
> >>> Since Linux guest also wants to do the same thing, let's making this
> mechanism to be a generic interface!
> >>> I'll post a patch adding ioctl(SG_IO) support to xen-blkfront together
> with a updated version of this patch soon.
> >>
> >> I do not think this feature is generally useful outside of this
> >> unspecified use case.  I do not think that supplying details about
> >> underlying storage device (beyond generic properties) to guests is
> >> sensible (e.g., what if the guest snapshot is restored on different
> >> storage?).
> >
> > The restore process (xl) can update the XenStore key with the new
> storage.
> 
> And how is this going to be communicated to the application using the data?
> 
> >> And thus I do not not think we should either: a) make this part of the
> >> blkif ABI; or b) add support to xen-blkfront or xen-blkback.
> >
> > It is already coded in Windows PV drivers so I am not following why
> > codyfing this in the blkif.h is harmful?
> 
> My understanding was that this was a legacy hack that should be removed
> (we do not currently make use of it).
>

AFAIK XenServer still very much makes use of it.

  Paul
 
> David

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