[PATCH] arm64: runtime-const: save one instruction when ARM64_VA_BITS <= 48

[Catalin Marinas]

On Wed, Feb 25, 2026 at 10:46:13PM +0800, Jisheng Zhang wrote:
> Currently, the runtime_const_ptr() uses 4 instructions to move a long
> imm to GP, but when ARM64_VA_BITS <= 48(which is true for android and
> armbian), the top 8bits of runtime cont ptr is all '1', so we can make
                    ^^^^^
                    8 or 16?

> use of the movn instruction to construct the imm's top 8bits and lower
> 16bits at the same time, thus save one instruction.

This works as long as KASAN_{SW,HW}_TAGS is disabled, otherwise the top
byte of a pointer is not guaranteed to be 0xff. I think both
filename_init() and dcache_init() can pass tagged pointers.

> diff --git a/arch/arm64/include/asm/runtime-const.h b/arch/arm64/include/asm/runtime-const.h
> index be5915669d23..6797dd37d690 100644
> --- a/arch/arm64/include/asm/runtime-const.h
> +++ b/arch/arm64/include/asm/runtime-const.h
> @@ -7,6 +7,8 @@
>  /* Sigh. You can still run arm64 in BE mode */
>  #include <asm/byteorder.h>
>  
> +#if CONFIG_ARM64_VA_BITS > 48

You could use VA_BITS, it's shorter, though if you add the KASAN checks
it's a pretty long #if to copy all over the place. We could untag the
pointer but it kind of defeats the purpose of enabling KASAN in the
first place.

Given that Android enables KASAN_HW_TAGS by default, not sure we should
bother with this change. Do you have any perf data to show that it's
worth it?

-- 
Catalin