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Date: Wed, 29 Apr 2026 09:57:10 +0200
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Subject: Re: [PATCH v4 01/15] arm64: mm: Map the linear alias of text/rodata
 as tagged
To: Ard Biesheuvel <ardb@kernel.org>, Ard Biesheuvel <ardb+git@google.com>,
 linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org, Will Deacon <will@kernel.org>,
 Catalin Marinas <catalin.marinas@arm.com>,
 Mark Rutland <mark.rutland@arm.com>, Ryan Roberts <ryan.roberts@arm.com>,
 Anshuman Khandual <anshuman.khandual@arm.com>,
 Liz Prucka <lizprucka@google.com>, Seth Jenkins <sethjenkins@google.com>,
 Kees Cook <kees@kernel.org>, Mike Rapoport <rppt@kernel.org>,
 David Hildenbrand <david@kernel.org>,
 Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
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References: <20260427153416.2103979-17-ardb+git@google.com>
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From: Kevin Brodsky <kevin.brodsky@arm.com>
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On 28/04/2026 18:23, Ard Biesheuvel wrote:
> Hi Kevin,
>
> On Tue, 28 Apr 2026, at 16:16, Kevin Brodsky wrote:
>> On 27/04/2026 17:34, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>
>>> Before moving the empty_zero_page into the __ro_after_init section, make
>>> sure it has the memory-tagged type. This is needed to ensure that
>>> cpu_enable_mte() will be able to initialize the tags correctly.
>> mark_linear_text_alias_ro() is called after all features have been
>> detected and enabled, so do we actually need this?
>>
>> AFAICT the lines that matter for cpu_enable_mte() are those, in map_mem():
>>
>>     __map_memblock(pgdp, kernel_start, kernel_end,
>>                PAGE_KERNEL, NO_CONT_MAPPINGS);
>>
>> It is probably this call that needs to be changed to use
>> pgprot_tagged(PAGE_KERNEL).
>>
> I see. I guess we should probably update both, no?

We could, but would we ever access the tags in data mapped read-only?
cpu_enable_mte() needs tag access to zero them, but AFAIU it's really a
special case that is only relevant to the situation where the data is RW.

- Kevin