linux-kernel - Re: [PATCH v4 3/4] arm64/mm: Move PTE_PRESENT_INVALID to overlay PTE

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Message-ID: <d6763648-86fd-40d2-b261-eb59a4721d91@redhat.com>
Date: Tue, 7 May 2024 13:43:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, 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>, Peter Xu <peterx@...hat.com>,
 Mike Rapoport <rppt@...ux.ibm.com>, Shivansh Vij <shivanshvij@...look.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/4] arm64/mm: Move PTE_PRESENT_INVALID to overlay
 PTE_NG

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@....com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb