Messages in this thread | | | Date | Mon, 29 Apr 2024 17:34:01 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH v2 2/3] arm64/mm: Move PTE_INVALID to overlay PTE_NS |
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On Mon, Apr 29, 2024 at 03:02:06PM +0100, Ryan Roberts wrote: > PTE_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_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_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_INVALID to a low bit we can lay the swap pte out so that the > offset field could grow to 53 bits in future. > > So let's move PTE_INVALID to overlay PTE_NS (bit 5). PTE_NS is res0 for > SW outside of the secure state so Linux will never need to touch it. > > These are both marginal benefits, but make things a bit tidier in my > opinion. > > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
(subject to renaming PTE_INVALID to PTE_PRESENT_INVALID)
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