[PATCH 1/2] arm64: errata: Work around AmpereOne's erratum AC03_CPU_36

[Marc Zyngier]

On Tue, 15 Apr 2025 16:47:10 +0100,
D Scott Phillips <scott at os.amperecomputing.com> wrote:
> 
> AC03_CPU_36 can cause asynchronous exceptions to be routed to the wrong
> exception level if an async exception coincides with an update to the
> controls for the target exception level in HCR_EL2. On affected
> machines, always do writes to HCR_EL2 with async exceptions blocked.

From the actual errata document [1]:

<quote>
If an Asynchronous Exception to EL2 occurs, while EL2 software is
changing the EL2 exception control bits from a configuration where
asynchronous exceptions are routed to EL2 to a configuration where
asynchronous exceptions are routed to EL1, the processor may exhibit
the incorrect exception behavior of routing an interrupt taken at EL2
to EL1.  The affected system register is HCR_EL2, which contains
control bits for routing and enabling of EL2 exceptions.
</quote>

My reading is that things can go wrong when clearing the xMO bits.

I don't think we need to touch the xMO bits at all when running
VHE. So my preference would be to:

- simply leave the xMO bits set at all times (nothing bad can happen
  from that, can it?)

- prevent these systems from using anything but VHE (and fail KVM init
  otherwise)

This would save a lot of maintenance hassle and the extreme ugliness
of this patch.

Thanks,

	M.

[1] https://amperecomputing.com/assets/AmpereOne_Developer_ER_v0_80_20240823_28945022f4.pdf

-- 
Without deviation from the norm, progress is not possible.