linux-kernel - Re: [PATCH v2] arm64: mm: Move KPTI helpers to mmu.c

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Message-ID: <ddc84d24-8bb6-48ca-9cab-cee8d4e0eca6@arm.com>
Date: Thu, 18 Sep 2025 09:24:06 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Ryan Roberts <ryan.roberts@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Catalin Marinas <catalin.marinas@....com>, Kees Cook <kees@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Yeoreum Yun <yeoreum.yun@....com>
Subject: Re: [PATCH v2] arm64: mm: Move KPTI helpers to mmu.c

On 17/09/2025 17:35, Will Deacon wrote:
> Hey Kevin,
>
> On Fri, Sep 12, 2025 at 08:39:08AM +0100, Kevin Brodsky wrote:
>> create_kpti_ng_temp_pgd() is currently defined (as an alias) in
>> mmu.c without matching declaration in a header; instead cpufeature.c
>> makes its own declaration. This is clearly not pretty, and as commit
>> ceca927c86e6 ("arm64: mm: Fix CFI failure due to kpti_ng_pgd_alloc
>> function signature") showed, it also makes it very easy for the
>> prototypes to go out of sync.
>>
>> All this would be much simpler if kpti_install_ng_mappings() and
>> associated functions lived in mmu.c, where they logically belong.
>> This is what this patch does:
>> - Move kpti_install_ng_mappings() and associated functions from
>>   cpufeature.c to mmu.c, add a declaration to <asm/mmu.h>
>> - Remove create_kpti_ng_temp_pgd() and just call
>>   __create_pgd_mapping_locked() directly instead
>> - Mark all these functions __init
>> - Move __initdata after kpti_ng_temp_alloc (as suggested by
>>   checkpatch)
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
>> Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>> v1..v2:
>> * Removed create_kpti_ng_temp_pgd() instead of making it a wrapper
>>   [Ryan's suggestion]
>> * Added Reviewed-by's.
>> ---
>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>> Cc: Ard Biesheuvel <ardb@...nel.org>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Kees Cook <kees@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Yeoreum Yun <yeoreum.yun@....com>
>> ---
>>  arch/arm64/include/asm/mmu.h   |  6 +++
>>  arch/arm64/kernel/cpufeature.c | 97 ----------------------------------
>>  arch/arm64/mm/mmu.c            | 96 +++++++++++++++++++++++++++++----
>>  3 files changed, 93 insertions(+), 106 deletions(-)
> Thanks, this looks fine to me. However, it conflicts with ceca927c86e6
> ("arm64: mm: Fix CFI failure due to kpti_ng_pgd_alloc function
> signature") which landed after -rc1 so I'll either queue this late
> (after merging in for-next/fixes to for-next/core) or we can defer it to
> -rc1.

Ah yes, this patch is based on -rc5 so it depends on that commit (also
mentioned in the commit message above). Didn't realise it caused trouble
for merging!

> If we get to -rc2 and it's not in Linus' tree, then please repost
> because it means I forgot about it :)

Sure I'll prod you if I don't see it by then - I don't think I'll need
to repost a new version though, unless that code changes again in the
meantime.

- Kevin