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.
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