linux-kernel - Re: [PATCH RESEND v7 6/6] arm64: futex: support futex with FEAT

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Message-ID: <aMRTu3lcwqhu-dYY@arm.com>
Date: Fri, 12 Sep 2025 18:09:15 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: will@...nel.org, broonie@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, james.morse@....com,
	ardb@...nel.org, scott@...amperecomputing.com,
	suzuki.poulose@....com, yuzenghui@...wei.com, mark.rutland@....com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v7 6/6] arm64: futex: support futex with FEAT_LSUI

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@....com>