It recently appeared that, when running VHE, there is a notable
difference between using CNTKCTL_EL1 and CNTHCTL_EL2, despite what
the architecture documents:

- When accessed from EL2, bits [19:18] and [16:10] of CNTKCTL_EL1 have
  the same assignment as CNTHCTL_EL2
- When accessed from EL1, bits [19:18] and [16:10] are RES0

It is all OK, until you factor in NV, where the EL2 guest runs at EL1.
In this configuration, CNTKCTL_EL11 doesn't trap, nor ends up in
the VNCR page. This means that any write from the guest affecting
CNTHCTL_EL2 using CNTKCTL_EL1 ends up losing some state. Not good.

The fix it obvious: don't use CNTKCTL_EL1 if you want to change bits
that are not part of the EL1 definition of CNTKCTL_EL1, and use
CNTHCTL_EL2 instead. This doesn't change anything for a bare-metal OS,
and fixes it when running under NV. The NV hypervisor will itself
have to work harder to merge the two accessors.

Note that there is a pending update to the architecture to address
this issue by making the affected bits UNKNOWN when CNTKCTL_EL1 is
used from EL2 with VHE enabled.

Fixes: c605ee24 ("KVM: arm64: timers: Allow physical offset without CNTPOFF_EL2")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org # v6.4
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20230627140557.544885-1-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>