linux-kernel - Re: [PATCH v2 2/3] arm64/mm: Move PTE_INVALID to overlay PTE

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Date: Mon, 29 Apr 2024 17:34:01 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>, Joey Gouly <joey.gouly@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>,
	Mike Rapoport <rppt@...ux.ibm.com>,
	Shivansh Vij <shivanshvij@...look.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] arm64/mm: Move PTE_INVALID to overlay PTE_NS

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@....com>

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

(subject to renaming PTE_INVALID to PTE_PRESENT_INVALID)