Re: [PATCH] arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/4/25 00:06, Kornel Dulęba wrote:
> When the 52-bit virtual addressing was enabled the select like
> ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
> Because of that the rnd max bits would be set to the default value of
> 18 when ARM64_VA_BITS=52.
> Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
> be used if 48-bit addressing was used. That's because the 52-bit
> addressing is used only if the caller provides a hint to mmap, with a
> fallback to 48-bit addressing.

Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
bits VA in case the user does request for 52 bit VA via mmap() hint and
the HW supports it ?

> 
> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
> Signed-off-by: Kornel Dulęba <korneld@google.com>
> ---
>  arch/arm64/Kconfig | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 748c34dc953c..38e0bac567f5 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
>  	default 24 if ARM64_VA_BITS=39
>  	default 27 if ARM64_VA_BITS=42
>  	default 30 if ARM64_VA_BITS=47
> -	default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
> -	default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
> -	default 33 if ARM64_VA_BITS=48
> +	default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
> +	default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
> +	default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
>  	default 14 if ARM64_64K_PAGES
>  	default 16 if ARM64_16K_PAGES
>  	default 18