Re: [PATCH v4 05/12] mm: introduce CONFIG_ARCH_HAS_LAZY_MMU_MODE

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 05/11/2025 04:40, Ritesh Harjani (IBM) wrote:
> Kevin Brodsky <kevin.brodsky@arm.com> writes:
>
>> Architectures currently opt in for implementing lazy_mmu helpers by
>> defining __HAVE_ARCH_ENTER_LAZY_MMU_MODE.
>>
>> In preparation for introducing a generic lazy_mmu layer that will
>> require storage in task_struct, let's switch to a cleaner approach:
>> instead of defining a macro, select a CONFIG option.
>>
>> This patch introduces CONFIG_ARCH_HAS_LAZY_MMU_MODE and has each
>> arch select it when it implements lazy_mmu helpers.
>> __HAVE_ARCH_ENTER_LAZY_MMU_MODE is removed and <linux/pgtable.h>
>> relies on the new CONFIG instead.
>>
>> On x86, lazy_mmu helpers are only implemented if PARAVIRT_XXL is
>> selected. This creates some complications in arch/x86/boot/, because
>> a few files manually undefine PARAVIRT* options. As a result
>> <asm/paravirt.h> does not define the lazy_mmu helpers, but this
>> breaks the build as <linux/pgtable.h> only defines them if
>> !CONFIG_ARCH_HAS_LAZY_MMU_MODE. There does not seem to be a clean
>> way out of this - let's just undefine that new CONFIG too.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
>> ---
>>  arch/arm64/Kconfig                                 | 1 +
>>  arch/arm64/include/asm/pgtable.h                   | 1 -
>>  arch/powerpc/include/asm/book3s/64/tlbflush-hash.h | 2 --
>>  arch/powerpc/platforms/Kconfig.cputype             | 1 +
>>  arch/sparc/Kconfig                                 | 1 +
>>  arch/sparc/include/asm/tlbflush_64.h               | 2 --
>>  arch/x86/Kconfig                                   | 1 +
>>  arch/x86/boot/compressed/misc.h                    | 1 +
>>  arch/x86/boot/startup/sme.c                        | 1 +
>>  arch/x86/include/asm/paravirt.h                    | 1 -
>>  include/linux/pgtable.h                            | 2 +-
>>  mm/Kconfig                                         | 3 +++
>>  12 files changed, 10 insertions(+), 7 deletions(-)
> Maybe we can add this to ... ?
>
> Documentation/features/vm/lazy_mmu/arch-support.txt
>
> #
> # Feature name:          lazy_mmu mode
> #         Kconfig:       ARCH_HAS_LAZY_MMU_MODE
> #         description:   arch supports arch_{enter|flush|leave}_lazy_mmu_mode()
> #
>     -----------------------
>     |         arch |status|
>     -----------------------
>     |       arm64: |  ok  |
>     |     powerpc: |  ok  |
>     |       sparc: |  ok  |
>     |         x86: |  ok  |
>     -----------------------

That's an interesting idea but I'm not sure it really makes sense for
lazy MMU? AFAIU these arch-support.txt files are meant to help identify
which generic features an arch has support for. Lazy MMU isn't really a
feature though, in the sense that what it does is entirely defined by
the arch. This patch does introduce a generic layer, but ultimately it
remains a collection of arch hooks.

> As for this patch, the changes are mostly straight forward around the
> configs part. This looks good to me. Please feel free to add: 
>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Thanks for the review!

- Kevin