linux-kernel - Re: [PATCH 2/2] riscv: introduce percpu.h into include/asm

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Message-ID: <e7653089-5c57-ee61-ed01-0b3245e10d82@gentwo.org>
Date: Wed, 20 Aug 2025 16:26:45 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Yunhui Cui <cuiyunhui@...edance.com>
cc: yury.norov@...il.com, linux@...musvillemoes.dk, paul.walmsley@...ive.com, 
    palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr, 
    linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
    dennis@...nel.org, tj@...nel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] riscv: introduce percpu.h into include/asm

On Tue, 19 Aug 2025, Yunhui Cui wrote:

> +#define __PERCPU_AMO_OP_CASE(sfx, name, sz, amo_insn)			\
> +static inline void							\
> +__percpu_##name##_amo_case_##sz(void *ptr, unsigned long val)		\
> +{									\
> +	asm volatile (							\
> +		"amo" #amo_insn #sfx " zero, %[val], %[ptr]"		\
> +		: [ptr] "+A" (*(u##sz *)ptr)				\
> +		: [val] "r" ((u##sz)(val))				\
> +		: "memory");						\
> +}

AMO creates a single instruction that performs the operation?

> +#define _pcp_protect(op, pcp, ...)					\
> +({									\
> +	preempt_disable_notrace();					\
> +	op(raw_cpu_ptr(&(pcp)), __VA_ARGS__);				\
> +	preempt_enable_notrace();					\
> +})

Is "op" a single instruction? If so then preempt disable / endable would
not be needed if there is no other instruction created.

But raw_cpu_ptr performs a SHIFT_PERCPU_PTR which performs an addition.
So you need the disabling of preemption to protect the add.

Is there a way on RISC V to embedd the pointer arithmetic in the "AMO"
instruction? Or can you use relative addressing to a register that
contains the cpu offset. I believe RISC V has a thread pointer?

If you can do this then a lot of preempt_enable/disable points can be
removed from the core kernel and the instruction may be as scalable as x86
which can do the per cpu operations with a single instruction.

> +
> +#define _pcp_protect_return(op, pcp, args...)				\
> +({									\
> +	typeof(pcp) __retval;						\
> +	preempt_disable_notrace();					\
> +	__retval = (typeof(pcp))op(raw_cpu_ptr(&(pcp)), ##args);	\
> +	preempt_enable_notrace();					\
> +	__retval;							\
> +})

Same here.