PTE_PRESENT_INVALID was previously occupying bit 59, which when a PTE is
valid can either be IGNORED, PBHA[0] or AttrIndex[3], depending on the
HW configuration. In practice this is currently not a problem because
PTE_PRESENT_INVALID can only be 1 when PTE_VALID=0 and upstream Linux
always requires the bit set to 0 for a valid pte.

However, if in future Linux wants to use the field (e.g. AttrIndex[3])
then we could end up with confusion when PTE_PRESENT_INVALID comes along
and corrupts the field - we would ideally want to preserve it even for
an invalid (but present) pte.

The other problem with bit 59 is that it prevents the offset field of a
swap entry within a swap pte from growing beyond 51 bits. By moving
PTE_PRESENT_INVALID to a low bit we can lay the swap pte out so that the
offset field could grow to 52 bits in future.

So let's move PTE_PRESENT_INVALID to overlay PTE_NG (bit 11).

There is no need to persist NG for a present-invalid entry; it is always
set for user mappings and is not used by SW to derive any state from the
pte. PTE_NS was considered instead of PTE_NG, but it is RES0 for
non-secure SW, so there is a chance that future architecture may
allocate the bit and we may therefore need to persist that bit for
present-invalid ptes.

These are both marginal benefits, but make things a bit tidier in my
opinion.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20240503144604.151095-4-ryan.roberts@arm.comSigned-off-by: Will Deacon <will@kernel.org>