linux-kernel - Re: [PATCH 2/2] arm64: Avoid memcpy() for syscall_get

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Message-ID: <20251127143557.GA2591@strace.io>
Date: Thu, 27 Nov 2025 16:35:57 +0200
From: "Dmitry V. Levin" <ldv@...ace.io>
To: Jinjie Ruan <ruanjinjie@...wei.com>
Cc: linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
	chris@...kel.net, jcmvbkbc@...il.com, akpm@...ux-foundation.org,
	macro@...am.me.uk, charlie@...osinc.com, deller@....de,
	rostedt@...dmis.org, tglx@...utronix.de,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arm64: Avoid memcpy() for syscall_get_arguments()

On Thu, Nov 27, 2025 at 08:36:30PM +0800, Jinjie Ruan wrote:
> Do not use memcpy() to extract syscall arguments from struct pt_regs
> but rather just perform direct assignments.
> 
> The performance benchmarks with Generic Entry patch[1] with audit on
> from perf bench basic syscall on kunpeng920 gives roughly a 1%
> performance uplift and also aligns the implementation with
> x86 and RISC-V.
> 
> | Metric     | W/O this patch | With this patch | Change    |
> | ---------- | -------------- | --------------- | --------- |
> | Total time | 2.241 [sec]    | 2.211 [sec]     |  ↓1.36%   |
> | usecs/op   | 0.224157       | 0.221146        |  ↓1.36%   |
> | ops/sec    | 4,461,157      | 4,501,409       |  ↑0.9%    |
> 
> Before:
> <syscall_get_arguments.constprop.0>:
>        aa0103e2        mov     x2, x1
>        91002003        add     x3, x0, #0x8
>        f9408804        ldr     x4, [x0, #272]
>        f8008444        str     x4, [x2], #8
>        a9409404        ldp     x4, x5, [x0, #8]
>        a9009424        stp     x4, x5, [x1, #8]
>        a9418400        ldp     x0, x1, [x0, #24]
>        a9010440        stp     x0, x1, [x2, #16]
>        f9401060        ldr     x0, [x3, #32]
>        f9001040        str     x0, [x2, #32]
>        d65f03c0        ret
>        d503201f        nop
> 
> After:
>        a9408e82        ldp     x2, x3, [x20, #8]
>        2a1603e0        mov     w0, w22
>        f9400e84        ldr     x4, [x20, #24]
>        f9408a81        ldr     x1, [x20, #272]
>        9401c4ba        bl      ffff800080215ca8 <__audit_syscall_entry>
> 
> [1]: https://lore.kernel.org/all/20251126071446.3234218-1-ruanjinjie@huawei.com/
> Signed-off-by: Jinjie Ruan <ruanjinjie@...wei.com>
> ---
>  arch/arm64/include/asm/syscall.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
> index f3853047c28e..f3564ba97f7e 100644
> --- a/arch/arm64/include/asm/syscall.h
> +++ b/arch/arm64/include/asm/syscall.h
> @@ -82,9 +82,11 @@ static inline void syscall_get_arguments(struct task_struct *task,
>  					 unsigned long *args)
>  {
>  	args[0] = regs->orig_x0;
> -	args++;
> -
> -	memcpy(args, &regs->regs[1], 5 * sizeof(args[0]));
> +	args[1] = regs->regs[1];
> +	args[2] = regs->regs[2];
> +	args[3] = regs->regs[3];
> +	args[4] = regs->regs[4];
> +	args[5] = regs->regs[5];
>  }
>  
>  static inline void syscall_set_arguments(struct task_struct *task,

Please keep syscall_get_arguments() and syscall_set_arguments() in sync:
if you replace memset() with direct assignments in one of these functions,
please mirror the change in another.


-- 
ldv