Currently the PMD_PRESENT_INVALID and PTE_PROT_NONE functionality
explicitly occupy 2 bits in the PTE when PTE_VALID/PMD_SECT_VALID is
clear. This has 2 significant consequences:
- PTE_PROT_NONE consumes a precious SW PTE bit that could be used for
other things.
- The swap pte layout must reserve those same 2 bits and ensure they
are both always zero for a swap pte. It would be nice to reclaim at
least one of those bits.
Note that while PMD_PRESENT_INVALID technically only applies to pmds,
the swap pte layout is common to ptes and pmds so we are currently
effectively reserving that bit at both levels.
Let's replace PMD_PRESENT_INVALID with a more generic PTE_INVALID bit,
which occupies the same position (bit 59) but applies uniformly to
page/block descriptors at any level. This bit is only interpretted when
PTE_VALID is clear. If it is set, then the pte is still considered
present; pte_present() returns true and all the fields in the pte follow
the HW interpretation (e.g. SW can safely call pte_pfn(), etc). But
crucially, the HW treats the pte as invalid and will fault if it hits.
With this in place, we can remove PTE_PROT_NONE entirely and instead
represent PROT_NONE as a present but invalid pte (PTE_VALID=0,
PTE_INVALID=1) with PTE_USER=0 and PTE_UXN=1. This is a unique
combination that is not used anywhere else.
The net result is a clearer, simpler, more generic encoding scheme that
applies uniformly to all levels. Additionally we free up a PTE SW bit a
swap pte bit (bit 58 in both cases).
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>