Re: [PATCH v5 01/10] mm: x86, arm64: add arch_has_hw_pte_young()

[Will Deacon <will@kernel.org>]

On Wed, Nov 10, 2021 at 09:15:01PM -0700, Yu Zhao wrote:
> Some architectures automatically set the accessed bit in PTEs, e.g.,
> x86 and arm64 v8.2. On architectures that do not have this capability,
> clearing the accessed bit in a PTE triggers a page fault following the
> TLB miss of this PTE.
> 
> Being aware of this capability can help make better decisions, i.e.,
> whether to limit the size of each batch of PTEs and the burst of
> batches when clearing the accessed bit.
> 
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
> ---
>  arch/arm64/include/asm/cpufeature.h |  5 +++++
>  arch/arm64/include/asm/pgtable.h    | 13 ++++++++-----
>  arch/arm64/kernel/cpufeature.c      | 10 ++++++++++
>  arch/arm64/tools/cpucaps            |  1 +
>  arch/x86/include/asm/pgtable.h      |  6 +++---
>  include/linux/pgtable.h             | 13 +++++++++++++
>  mm/memory.c                         | 14 +-------------
>  7 files changed, 41 insertions(+), 21 deletions(-)

*Please* cc the maintainers on arch patches. I asked you that last time,
too:

https://lore.kernel.org/r/20210819091923.GA15467@willie-the-truck

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 6ec7036ef7e1..940615d33845 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2157,6 +2157,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.matches = has_hw_dbm,
>  		.cpu_enable = cpu_enable_hw_dbm,
>  	},
> +	{
> +		.desc = "Hardware update of the Access flag",
> +		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.capability = ARM64_HW_AF,
> +		.sys_reg = SYS_ID_AA64MMFR1_EL1,
> +		.sign = FTR_UNSIGNED,
> +		.field_pos = ID_AA64MMFR1_HADBS_SHIFT,
> +		.min_field_value = 1,
> +		.matches = has_cpuid_feature,
> +	},

As before, please don't make this a system feature as it will prohibit
onlining of late CPUs with mismatched access flag support and I really
don't see that being necessary.

You should just be able to use arch_faults_on_old_pte() as-is.

Will