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

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

[Jisheng Zhang]

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

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
---
 arch/arm64/include/asm/runtime-const.h | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

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
+
 #define runtime_const_ptr(sym) ({				\
 	typeof(sym) __ret;					\
 	asm_inline("1:\t"					\
@@ -20,6 +22,22 @@
 		:"=r" (__ret));					\
 	__ret; })
 
+#else
+
+#define runtime_const_ptr(sym) ({				\
+	typeof(sym) __ret;					\
+	asm_inline("1:\t"					\
+		"movn %0, #0x3210\n\t"				\
+		"movk %0, #0x89ab, lsl #16\n\t"			\
+		"movk %0, #0x4567, lsl #32\n\t"			\
+		".pushsection runtime_ptr_" #sym ",\"a\"\n\t"	\
+		".long 1b - .\n\t"				\
+		".popsection"					\
+		: "=r" (__ret));				\
+	__ret; })
+
+#endif
+
 #define runtime_const_shift_right_32(val, sym) ({		\
 	unsigned long __ret;					\
 	asm_inline("1:\t"					\
@@ -58,11 +76,19 @@ static inline void __runtime_fixup_caches(void *where, unsigned int insns)
 static inline void __runtime_fixup_ptr(void *where, unsigned long val)
 {
 	__le32 *p = lm_alias(where);
+#if CONFIG_ARM64_VA_BITS > 48
 	__runtime_fixup_16(p, val);
+#else
+	__runtime_fixup_16(p, ~val);
+#endif
 	__runtime_fixup_16(p+1, val >> 16);
 	__runtime_fixup_16(p+2, val >> 32);
+#if CONFIG_ARM64_VA_BITS > 48
 	__runtime_fixup_16(p+3, val >> 48);
 	__runtime_fixup_caches(where, 4);
+#else
+	__runtime_fixup_caches(where, 3);
+#endif
 }
 
 /* Immediate value is 6 bits starting at bit #16 */
-- 
2.51.0

Re: [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

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

[Jisheng Zhang]

On Fri, Feb 27, 2026 at 04:34:04PM +0000, Catalin Marinas wrote:
> 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?

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.

oops, you are right! I missed both: KASAN_SW_TAGS is disabled due to
overhead while KASAN_HW_TAGS doesn't work since I don't have the
platform. Will take care these two options in the future.
> 
> > 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.

Usually, the runtime const ptr is set once during boot then read onlly
so IMHO we don't need KASAN to catch the ptr related memory bugs.

> 
> 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?

Good question. I guess a micro benchmark just measure the 4 instructions
vs 3 instructions thus 25% saving can't persuade you to merge it. Let me
find or write a userspace program to iterate a deep directory to show
the improvement. Any hint is appreciated.

Thanks