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

[David Hildenbrand]

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