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KVM allows userspace to write an IMPDEF PMU version to the corresponding
32bit and 64bit ID register fields for the sake of backwards
compatibility with kernels that lacked commit 3d0dba57 ("KVM: arm64:
PMU: Move the ID_AA64DFR0_EL1.PMUver limit to VM creation"). Plumbing
that IMPDEF PMU version through to the gues is getting in the way of
progress, and really doesn't any sense in the first place.

Bite the bullet and reinterpret the IMPDEF PMU version as NI (0) for
userspace writes. Additionally, spill the dirty details into a comment.

Link: https://lore.kernel.org/r/20230609190054.1542113-5-oliver.upton@linux.dev

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>