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

[Christoph Hellwig]

On Sun, Apr 23, 2017 at 10:20:59AM +0300, Sagi Grimberg wrote:
> Christoph,
>
>> All our block I/O must be cache line aligned, so dword aligned SGLs
>> should not be an issue.  And for PRPs or MRs we'll always use the
>> host page size.
>
> Where is that constraint coming from? Is that new? Back when I wrote
> GAP support in rdma I tested a single byte alignment via vectored
> direct IO and it seemed to be going through...

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.

> 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);

still would fit NVMe dword-alignment SGLs, though :)