Re: [PATCH v4 3/4] arm64/mm: Move PTE_PRESENT_INVALID to overlay PTE_NG

[David Hildenbrand]

On 03.05.24 16:46, Ryan Roberts wrote:
> 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@xxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb