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git2svn
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2021-07-31T00:23:23.174529Z
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V 869
NVME: Multiple busdma related fixes.

- in nvme_qpair_process_completions() do dma sync before completion buffer
  is used.
- in nvme_qpair_submit_tracker(), don't do explicit wmb() also for arm
  and arm64. Bus_dmamap_sync() on these architectures is sufficient to ensure
  that all CPU stores are visible to external (including DMA) observers.
- Allocate completion buffer as BUS_DMA_COHERENT. On not-DMA coherent systems,
  buffers continuously owned (and accessed) by DMA must be allocated with this
  flag. Note that BUS_DMA_COHERENT flag is no-op on DMA coherent systems
  (or coherent buses in mixed systems).

MFC after:	4 weeks
Reviewed by:	mav, imp
Differential Revision: https://reviews.freebsd.org/D27446

(cherry picked from commit 8f9d5a8dbf4ea69c5f9a1e3a36e23732ffaa5c75)

Git Hash:   696065e4f21ad9bb52def38726c2f4ed593a2602
Git Author: mmel@FreeBSD.org
END