[PATCH v2 3/4] nvme: enable SG gaps support

[Max Gurtovoy]

On 4/23/2017 12:45 PM, Sagi Grimberg wrote:
>
>> Each block queue has a ->dma_alignment field, which is set to 511
>> by default unless changed.  Everything not aligned to that will be
>> bounced in __blk_rq_map_user_iov and friends before we send it to
>> the driver.
>
> Thanks, I missed the default setting...
>
>>> I also just tested Bart's unaligned test [1] on scsi_debug and it seems
>>> to go through as well (alignment and length are 4)...
>>
>> SCSI reduces the alignment in __scsi_init_queue:
>>
>>     /*
>>          * set a reasonable default alignment on word boundaries: the
>>      * host and device may alter it using
>>      * blk_queue_update_dma_alignment() later.
>>      */
>>     blk_queue_dma_alignment(q, 0x03);
>
> That explains why I was able to test that with iSER :)
>
> I guess I didn't test single byte alignment after all, but
> I definitely remember using less than a sector...
>
>> still would fit NVMe dword-alignment SGLs, though :)
>
> I would say its useful to have, but its only relevant to ioctls as
> normal io we explicitly on sub logical block size alignment.
>
> Anyways, I still think having ctrl->virt_boundary instead of
> ctrl->sg_gap_support is cleaner...

Sure, I tought about it and actually started my implementation like 
that. It wasn't so clean, though.