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Re: [Xen-devel] [RFC PATCH] Data integrity extension support for xen-block

On 04/08/2016 10:32 PM, David Vrabel wrote:
> On 08/04/16 15:20, Ian Jackson wrote:
>> David Vrabel writes ("Re: [RFC PATCH] Data integrity extension support for 
>> xen-block"):
>>> You need to read the relevant SCSI specification and find out what
>>> interfaces and behaviour the hardware has so you can specify compatible
>>> interfaces in blkif.
>>> My (brief) reading around this suggests that the integrity data has a
>>> specific format (a CRC of some form) and the integrity data written for
>>> sector S and retrieved verbatim when sector S is re-read.
>> I think it's this:
>> https://en.wikipedia.org/wiki/Data_Integrity_Field
>> https://www.kernel.org/doc/Documentation/block/data-integrity.txt
>> In which case AFAICT the format is up to the guest (ie the operating
>> system or file system) and it's opaque to the host (the storage) -
>> unless the guest consents, of course.
> I disagree, but I can't work out where to get the relevant T10 PI/DIF
> spec from to provide an authoritative link[1].  The DI metadata has as a
> set of well defined format, most of which include a 16-bit GUARD CRC, a
> 32 bit REFERENCE tag and 16 bit for user defined usage.


> The application cannot use all the bits for its own use since the
> hardware may check the GUARD and REFERENCE tags itself.
> David
> [0] Try: https://www.usenix.org/legacy/event/lsf07/tech/petersen.pdf

And https://oss.oracle.com/projects/data-integrity/dist/documentation/dix.pdf

No matter the actual format of the Integrity Meta Data looks like, it can be 
mapped to a scatter-list by using:
blk_rq_map_integrity_sg(struct request_queue *q, struct bio *bio, struct 
scatterlist *sglist)
just like blk_rq_map_sg(struct request_queue *q, struct request *rq, struct 
scatterlist *sglist) for normal data.

The extra scatter-list can be seen as the interface, we just need to find a 
good way transmitting this extra scatter-list between blkfront and blkback.


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