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

From: David Vrabel <david.vrabel@citrix.com>
To: Bob Liu <bob.liu@oracle.com>, Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: jgross@suse.com, xen-devel@lists.xen.org, annie.li@oracle.com,
	Paul.Durrant@citrix.com, David Vrabel <david.vrabel@citrix.com>,
	roger.pau@citrix.com
Subject: Re: [RFC PATCH] blkif.h: document scsi/0x12/0x83 node
Date: Tue, 22 Mar 2016 13:41:43 +0000	[thread overview]
Message-ID: <56F14B97.5050602@citrix.com> (raw)
In-Reply-To: <56F140C2.2050201@oracle.com>

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.

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.

> 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?).

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.

David

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