Re: [PATCH v4 01/11] mm: x86, arm64: add arch_has_hw_pte_young()

[Will Deacon <will@kernel.org>]

On Wed, Aug 18, 2021 at 12:30:57AM -0600, Yu Zhao wrote:
> Some architectures set the accessed bit in PTEs automatically, e.g.,
> x86, and arm64 v8.2 and later. On architectures that do not have this
> capability, clearing the accessed bit in a PTE triggers a page fault
> following the TLB miss.
> 
> 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>
> ---
>  arch/arm64/include/asm/cpufeature.h | 19 ++++++-------------
>  arch/arm64/include/asm/pgtable.h    | 10 ++++------
>  arch/arm64/kernel/cpufeature.c      | 19 +++++++++++++++++++
>  arch/arm64/mm/proc.S                | 12 ------------
>  arch/arm64/tools/cpucaps            |  1 +
>  arch/x86/include/asm/pgtable.h      |  6 +++---
>  include/linux/pgtable.h             | 12 ++++++++++++
>  mm/memory.c                         | 14 +-------------
>  8 files changed, 46 insertions(+), 47 deletions(-)

Please cc linux-arm-kernel and the maintainers on arm64 patches.

> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 9bb9d11750d7..2020b9e818c8 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -776,6 +776,12 @@ static inline bool system_supports_tlb_range(void)
>  		cpus_have_const_cap(ARM64_HAS_TLB_RANGE);
>  }
>  
> +/* Check whether hardware update of the Access flag is supported. */
> +static inline bool system_has_hw_af(void)
> +{
> +	return IS_ENABLED(CONFIG_ARM64_HW_AFDBM) && cpus_have_const_cap(ARM64_HW_AF);
> +}

How accurate does this need to be? Heterogeneous (big/little) systems are
very common on arm64, so the existing code enables hardware access flag
unconditionally on CPUs that support it, meaning we could end up running
on a system where some CPUs have hardware update and others do not.

With your change, we only enable hardware access flag if _all_ CPUs support
it (and furthermore, we prevent late onlining of CPUs without the feature
if was detected at boot). This sacrifices a lot of flexibility, particularly
if we end up tackling CPU errata in this area in future, and it's not clear
that it's really required for what you're trying to do.

Will