[Bug 1987541] Re: shim executes GRUB w/ dirty instruction cache on arm64

[dann frazier]

I rebooted a Bluefield 2 device running both jammy and focal in non-
Secure Boot mode several times without any issue (this was the
system/mode where we reproduced the issue before). These are the only
releases supported on this hardware (linux-bluefield only exists there).

As part of verifying bug 2004208, I rebooted kinetic, jammy, and focal
arm64 VMs in Secure Boot mode:

  https://bugs.launchpad.net/ubuntu/+source/shim-
signed/+bug/2004208/comments/7

I've also rebooted a bionic VM in non-SecureBoot mode w/o any issue. No
regressions were found.

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Title:
  shim executes GRUB w/ dirty instruction cache on arm64

Status in shim package in Ubuntu:
  Fix Released
Status in shim source package in Bionic:
  Fix Committed
Status in shim source package in Focal:
  Fix Committed
Status in shim source package in Jammy:
  Fix Committed
Status in shim source package in Kinetic:
  Fix Committed

Bug description:
  [Impact]
  On arm64 platforms, GRUB may occasionally crash after being executed by shim. We're seeing it on the order of 1/100 boots.

  [Test Case]
  Put an arm64 server in a reboot loop. We're seeing this w/ a Cortex A72-based system (Nvidia Bluefield).

  [Fix]
  https://github.com/rhboot/shim/commit/5c537b3d0cf8c393dad2e61d49aade68f3af1401

  [What could go wrong]
  The only negative here would seem to be the performance impact of flushing the cache, which is unlikely to be noticeable.

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