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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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.
> 
> 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?
> 
> > 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 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?


> 
> David
> 
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