From: Keith Busch <kbusch@xxxxxxxxxx> The typical journey a user address takes for a read or write to a block device undergoes various represenations for every IO. Each consumes memory and CPU cycles. When the backing storage is NVMe, the sequence looks something like the following: __user void * struct iov_iter struct pages[] struct bio_vec[] struct scatterlist[] __le64[] Applications will often use the same buffer for many IO, though, so these per-IO transformations to reach the exact same hardware descriptor is unnecessary. The io_uring interface already provides a way for users to register buffers to get to the 'struct bio_vec[]'. That still leaves the scatterlist needed for the repeated dma_map_sg(), then transform to nvme's PRP list format. This series takes the registered buffers a step further. A block driver can implement a new .dma_map() callback to complete the to the hardware's DMA mapped address representation, and return a cookie so a user can reference it later for any given IO. When used, the block stack can skip significant amounts of code, improving CPU utilization, and, if not bandwidth limited, IOPs. The larger the IO, the more signficant the improvement. The implementation is currently limited to mapping a registered buffer to a single block device. Here's some perf profiling 128k random read tests demonstrating the CPU savings: With premapped bvec: --46.84%--blk_mq_submit_bio | |--31.67%--blk_mq_try_issue_directly | --31.57%--__blk_mq_try_issue_directly | --31.39%--nvme_queue_rq | |--25.35%--nvme_prep_rq.part.68 With premapped DMA: --25.86%--blk_mq_submit_bio | |--12.95%--blk_mq_try_issue_directly | --12.84%--__blk_mq_try_issue_directly | --12.53%--nvme_queue_rq | |--5.01%--nvme_prep_rq.part.68 Keith Busch (5): blk-mq: add ops to dma map bvec iov_iter: introduce type for preregistered dma tags block: add dma tag bio type io_uring: add support for dma pre-mapping nvme-pci: implement dma_map support block/bdev.c | 20 +++ block/bio.c | 25 ++- block/blk-merge.c | 18 +++ drivers/nvme/host/pci.c | 291 +++++++++++++++++++++++++++++++++- include/linux/bio.h | 21 +-- include/linux/blk-mq.h | 25 +++ include/linux/blk_types.h | 6 +- include/linux/blkdev.h | 16 ++ include/linux/uio.h | 9 ++ include/uapi/linux/io_uring.h | 12 ++ io_uring/io_uring.c | 129 +++++++++++++++ io_uring/net.c | 2 +- io_uring/rsrc.c | 13 +- io_uring/rsrc.h | 16 +- io_uring/rw.c | 2 +- lib/iov_iter.c | 25 ++- 16 files changed, 600 insertions(+), 30 deletions(-) -- 2.30.2