On Mon, Jan 18, 2021 at 09:45:12AM +0000, Marc Zyngier wrote:

It recently came to light that there is a need to be able to override
some CPU features very early on, before the kernel is fully up and
running. The reasons for this range from specific feature support
(such as using Protected KVM on VHE HW, which is the main motivation
for this work) to errata workaround (a feature is broken on a CPU and
needs to be turned off, or rather not enabled).

This series tries to offer a limited framework for this kind of
problems, by allowing a set of options to be passed on the
command-line and altering the feature set that the cpufeature
subsystem exposes to the rest of the kernel. Note that this doesn't
change anything for code that directly uses the CPU ID registers.

The series completely changes the way a VHE-capable system boots, by
*always* booting non-VHE first, and then upgrading to VHE when deemed
capable. Although it sounds scary, this is actually simple to
implement (and I wish I had done that five years ago). The "upgrade to
VHE" path is then conditioned on the VHE feature not being disabled
from the command-line.

Said command-line parsing borrows a lot from the kaslr code, and
subsequently allows the "nokaslr" option to be moved to the new
infrastructure (though it all looks a bit... odd).

Further patches now add support for disabling BTI and PAuth, the
latter being based on an initial series by Srinivas Ramana[0]. There
is some ongoing discussions about being able to disable MTE, but no
clear resolution on that subject yet.

This has been tested on multiple VHE and non-VHE systems.

* From v3 [3]:
  - Fixed the VHE_RESTART stub (duh!)
  - Switched to using arm64_ftr_safe_value() instead of the user
    provided value
  - Per-feature override warning

* From v2 [2]:
  - Simplify the VHE_RESTART stub
  - Fixed a number of spelling mistakes, and hopefully introduced a
    few more
  - Override features in __read_sysreg_by_encoding()
  - Allow both BTI and PAuth to be overridden on the command line
  - Rebased on -rc3

* From v1 [1]:
  - Fix SPE init on VHE when EL2 doesn't own SPE
  - Fix re-init when KASLR is used
  - Handle the resume path
  - Rebased to 5.11-rc2

[0] https://lore.kernel.org/r/1610152163-16554-1-git-send-email-sramana@codeaurora.org (local)
[1] https://lore.kernel.org/r/20201228104958.1848833-1-maz@kernel.org (local)
[2] https://lore.kernel.org/r/20210104135011.2063104-1-maz@kernel.org (local)
[3] https://lore.kernel.org/r/20210111132811.2455113-1-maz@kernel.org (local)

Pretty cool! Left a few minor comments here and there, other than that:
Acked-by: David Brazdil <redacted>

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