Re: [PATCH RESEND v7 6/6] arm64: futex: support futex with FEAT_LSUI

Catalin Marinas
2025-09-12T17:09:19Z

On Sat, Aug 16, 2025 at 04:19:29PM +0100, Yeoreum Yun wrote:
> @@ -115,11 +117,137 @@ __llsc_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> return ret;
> }
>
> +#ifdef CONFIG_AS_HAS_LSUI
> +
> +#define __LSUI_PREAMBLE ".arch_extension lsui\n"
> +
> +#define LSUI_FUTEX_ATOMIC_OP(op, asm_op, mb) \
> +static __always_inline int \
> +__lsui_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
> +{ \
> + int ret = 0; \
> + int oldval; \
> + \
> + uaccess_ttbr0_enable(); \

I think we can drop uaccess_ttbr0_*() from these functions. At the
kconfig level, TTBR0_PAN selects PAN. Hardware with LSUI will also
have PAN (since 8.1), so the above is an unnecessary branch or nop,
depending on how the alternatives play out. But add a comment instead.

> +static __always_inline int
> +__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval)
> +{
> + unsigned int loops = LLSC_MAX_LOOPS;
> + int ret, oldval, tmp;
> +
> + uaccess_ttbr0_enable();
> + /*
> + * there are no ldteor/stteor instructions...
> + */
> + asm volatile("// __lsui_futex_atomic_eor\n"
> + __LSUI_PREAMBLE
> +" prfm pstl1strm, %2\n"
> +"1: ldtxr %w1, %2\n"
> +" eor %w3, %w1, %w5\n"
> +"2: stltxr %w0, %w3, %2\n"
> +" cbz %w0, 3f\n"
> +" sub %w4, %w4, %w0\n"
> +" cbnz %w4, 1b\n"
> +" mov %w0, %w6\n"
> +"3:\n"
> +" dmb ish\n"
> + _ASM_EXTABLE_UACCESS_ERR(1b, 3b, %w0)
> + _ASM_EXTABLE_UACCESS_ERR(2b, 3b, %w0)
> + : "=&r" (ret), "=&r" (oldval), "+Q" (*uaddr), "=&r" (tmp),
> + "+r" (loops)
> + : "r" (oparg), "Ir" (-EAGAIN)
> + : "memory");
> + uaccess_ttbr0_disable();
> +
> + if (!ret)
> + *oval = oldval;
> +
> + return ret;
> +}

That's an unfortunate omission from the architecture.

> +#define __lsui_llsc_body(op, ...) \
> +({ \
> + alternative_has_cap_likely(ARM64_HAS_LSUI) ? \
> + __lsui_##op(__VA_ARGS__) : __llsc_##op(__VA_ARGS__); \
> +})
> +
> +#else /* CONFIG_AS_HAS_LSUI */
> +
> +#define __lsui_llsc_body(op, ...) __llsc_##op(__VA_ARGS__)
> +
> +#endif /* CONFIG_AS_HAS_LSUI */
> +
> +
> #define FUTEX_ATOMIC_OP(op) \
> static __always_inline int \
> __futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
> { \
> - return __llsc_futex_atomic_##op(oparg, uaddr, oval); \
> + return __lsui_llsc_body(futex_atomic_##op, oparg, uaddr, oval); \
> }

That's what I got confused about. It looks fine:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>