[PATCH v2 0/8] ublk: io_desc optimizations

This series is based on the "ublk: harden user buffer handling" series.

ublk passes the parameters of incoming I/O in memory shared between the
kernel ublk driver and userspace ublk server in struct ublksrv_io_desc.

The thread submitting the ublk I/O writes to the io_desc, while the ublk
server thread handling the I/O reads the io_desc. This basically
guarantees a cache miss on both threads for each ublk I/O. Avoid the
cache misses by writing the io_desc on the server thread in the kernel
before dispatching the I/O to userspace.

The size of each io_desc is currently fixed to 24 bytes, which has been
an obstacle to extending it with additional fields [1]. Additionally,
with multiple ublk server threads handling I/Os from the same ublk queue
(possible with UBLK_F_PER_IO_DAEMON or UBLK_F_BATCH_IO), false sharing
results from adjacent io_descs sharing the same cache line. Add a
UBLK_F_IO_DESC_SIZE feature allowing the ublk server to increase the
size of the io_descs for its ublk devices.

[1]: https://lore.kernel.org/linux-block/aV8QfvaNO5P6vOs6@fedora/

v2:
- Cap io_desc_size arbitrarily at 256 (Ming)
- Add kublk list support, selftest (Ming)
- Add Reviewed-by tags, Suggested-by tag (Ming)

v1: https://lore.kernel.org/linux-block/[email protected]/

Caleb Sander Mateos (8):
  ublk: consistently use u16 for queue and tag numbers
  ublk: remove struct ublk_zoned_report_desc's operation field
  ublk: split request validation from io_desc init
  ublk: initialize io_desc on daemon task
  ublk: add UBLK_F_IO_DESC_SIZE
  selftests: ublk: add support for --io_desc_size
  selftests: ublk: add UBLK_F_IO_DESC_SIZE test
  ublk: lift checks out of ublk_{,un}map_io()

 drivers/block/ublk_drv.c                     | 261 ++++++++++---------
 include/uapi/linux/ublk_cmd.h                |   5 +-
 tools/testing/selftests/ublk/Makefile        |   1 +
 tools/testing/selftests/ublk/kublk.c         |  30 ++-
 tools/testing/selftests/ublk/kublk.h         |   6 +-
 tools/testing/selftests/ublk/test_loop_08.sh |  25 ++
 6 files changed, 188 insertions(+), 140 deletions(-)
 create mode 100755 tools/testing/selftests/ublk/test_loop_08.sh

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2.54.0