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Date: Fri, 3 May 2024 13:01:04 +0100
From: Will Deacon <will@kernel.org>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Joey Gouly <joey.gouly@arm.com>, Ard Biesheuvel <ardb@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	David Hildenbrand <david@redhat.com>, Peter Xu <peterx@redhat.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Shivansh Vij <shivanshvij@outlook.com>,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/3] arm64/mm: Enable userfaultfd write-protect
Message-ID: <20240503120104.GA18156@willie-the-truck>
References: <20240501145419.1390363-1-ryan.roberts@arm.com>
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Hi Ryan,

On Wed, May 01, 2024 at 03:54:16PM +0100, Ryan Roberts wrote:
> This series adds uffd write-protect support for arm64.
> 
> Previous attempts to add uffd-wp (and soft-dirty) have failed because of a
> perceived lack of available PTE SW bits. However it actually turns out that
> there are 2 available but they are hidden. PTE_PROT_NONE was previously
> occupying a SW bit, but can be moved, freeing up the SW bit. Bit 63 is marked as
> "IGNORED" in the Arm ARM, but it does not currently indicate "reserved for SW
> use" like it does for the other SW bits. I've confirmed with the spec owner that
> this is an oversight; the bit is intended to be reserved for SW use and the spec
> will clarify this in a future update.
> 
> So now we have two spare bits; patch 3 enables uffd-wp on arm64, using the SW
> bit freed up by moving PTE_PROT_NONE. This leaves bit 63 spare for future use
> (e.g. soft-dirty - see RFC at [3] - or some other usage).

Thanks, I think this series looks really good now.

From your discussion with Anshuman, it sounds like you're going to post
a new version with the first patch split up so I'll wait for that.

Will