[Bug 234838] ena drop-outs on 12.0-RELEASE

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234838

--- Comment #22 from commit-hook at freebsd.org ---
A commit references this bug:

Author: mw
Date: Thu May  7 11:28:40 UTC 2020
New revision: 360777
URL: https://svnweb.freebsd.org/changeset/base/360777

Log:
  Optimize ENA Rx refill for low memory conditions

  Sometimes, especially when there is not much memory in the system left,
  allocating mbuf jumbo clusters (like 9KB or 16KB) can take a lot of time
  and it is not guaranteed that it'll succeed. In that situation, the
  fallback will work, but if the refill needs to take a place for a lot of
  descriptors at once, the time spent in m_getjcl looking for memory can
  cause system unresponsiveness due to high priority of the Rx task. This
  can also lead to driver reset, because Tx cleanup routine is being
  blocked and timer service could detect that Tx packets aren't cleaned
  up. The reset routine can further create another unresponsiveness - Rx
  rings are being refilled there, so m_getjcl will again burn the CPU.
  This was causing NVMe driver timeouts and resets, because network driver
  is having higher priority.

  Instead of 16KB jumbo clusters for the Rx buffers, 9KB clusters are
  enough - ENA MTU is being set to 9K anyway, so it's very unlikely that
  more space than 9KB will be needed.

  However, 9KB jumbo clusters can still cause issues, so by default the
  page size mbuf cluster will be used for the Rx descriptors. This can have a
  small (~2%) impact on the throughput of the device, so to restore
  original behavior, one must change sysctl "hw.ena.enable_9k_mbufs" to
  "1" in "/boot/loader.conf" file.

  As a part of this patch (important fix), the version of the driver
  was updated to v2.1.2.

  Submitted by:   cperciva
  Reviewed by:    Michal Krawczyk <mk at semihalf.com>
  Reviewed by:    Ido Segev <idose at amazon.com>
  Reviewed by:    Guy Tzalik <gtzalik at amazon.com>
  MFC after:      3 days
  PR:             225791, 234838, 235856, 236989, 243531
  Differential Revision: https://reviews.freebsd.org/D24546

Changes:
  head/sys/dev/ena/ena.c
  head/sys/dev/ena/ena.h
  head/sys/dev/ena/ena_sysctl.c
  head/sys/dev/ena/ena_sysctl.h

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