Re: [PATCH v4 2/4] arm64/mm: Remove PTE_PROT_NONE bit

From: David Hildenbrand
Date: Tue May 07 2024 - 07:41:48 EST


On 03.05.24 16:46, Ryan Roberts wrote:
Currently the PTE_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.

But PTE_PRESENT_INVALID, which since the previous patch, applies
uniformly to page/block descriptors at any level when PTE_VALID is
clear, can already give us most of what PTE_PROT_NONE requires: 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.

So let's remove PTE_PROT_NONE entirely and instead represent PROT_NONE
as a present but invalid pte (PTE_VALID=0, PTE_PRESENT_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
and a swap pte bit (bit 58 in both cases).

Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb