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[Qemu-devel] [PATCH v2 0/7] dma-helpers, scsi-block: use SG_IO for all I

From: Paolo Bonzini
Subject: [Qemu-devel] [PATCH v2 0/7] dma-helpers, scsi-block: use SG_IO for all I/O on scsi-block
Date: Mon, 23 May 2016 14:54:04 +0200

scsi-block uses the block layer for reads and writes in order to avoid
allocating bounce buffers as big as the transferred data.  We know how
to split a large transfer to multiple reads and writes, and thus we can
use scsi-disk.c's existing code to do I/O in multiple chunks (for non-s/g
SCSI hosts) or through the DMA helpers (for s/g SCSI hosts).

Unfortunately, this has the side effect of eating the SCSI status except
in the very few cases where we can convert an errno code back to a SCSI
status.  It puts a big wrench in persistent reservations support in the
guest, for example.

Luckily, splitting a large transfer into multiple SBC commands is just as
easy, and this is what the last patch does.  It takes the original CDB,
patches in a modified starting sector and sector count, and executes the
SCSI command through blk_aio_ioctl.  It is also easy to pass a QEMUIOVector
to SG_IO, so that s/g SCSI hosts keep the performance.

This rebases the patches on top of Eric's changes for byte-based
BlockBackend access and fixes a few bugs I knew about in the RFC.

Patches 1, 5 and 6 are new.


Paolo Bonzini (7):
  dma-helpers: change interface to byte-based
  dma-helpers: change BlockBackend to opaque value in DMAIOFunc
  scsi-disk: introduce a common base class
  scsi-disk: introduce dma_readv and dma_writev
  scsi-disk: add need_fua_emulation to SCSIDiskClass
  scsi-disk: introduce scsi_disk_req_check_error
  scsi-block: always use SG_IO

 dma-helpers.c        |  54 +++++--
 hw/block/nvme.c      |   6 +-
 hw/ide/ahci.c        |   6 +-
 hw/ide/core.c        |  20 ++-
 hw/ide/internal.h    |   6 +-
 hw/ide/macio.c       |   2 +-
 hw/scsi/scsi-disk.c  | 409 +++++++++++++++++++++++++++++++++++++--------------
 include/sysemu/dma.h |  20 +--
 trace-events         |   2 +-
 9 files changed, 371 insertions(+), 154 deletions(-)


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