arm64: MPAM2_EL2 and MPAMHCR_EL2 are left UNKNOWN under arm64.nompam

[Fuad Tabba]

Hi Xi, James, Catalin, Will,

I'm going through the features that can be disabled on the host
command line, checking that EL2 either traps them or leaves its own
controls in a defined state. That quick and easy GMID_EL1 fix came out
of the same pass [1] :)

Since 10f885d63a0e ("arm64: Add override for MPAM"), arm64.nompam
skips the MPAM2_EL2 and MPAMHCR_EL2 writes in finalise_el2_state, so
on an MPAM PE with EL3 implemented the trap controls in both keep
their reset values, which are architecturally UNKNOWN.

I was wondering whether that was deliberate. AFAICT, restoring the
writes would trap to EL3 wherever MPAM3_EL3.TRAPLOWER is still set, so
it may not be safe on the machines the override was added for. I have
a patch restoring them, but I learned my lesson.

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260825164219.3347229-1-fuad.tabba@xxxxxxxxx/