linux-kernel - Re: [PATCH 1/4] riscv: entry: Convert ret_from

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Message-ID: <alpine.DEB.2.21.2501252313210.27203@angie.orcam.me.uk>
Date: Sat, 25 Jan 2025 23:30:09 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Charlie Jenkins <charlie@...osinc.com>
cc: Paul Walmsley <paul.walmsley@...ive.com>, 
    Palmer Dabbelt <palmer@...belt.com>, Huacai Chen <chenhuacai@...nel.org>, 
    WANG Xuerui <kernel@...0n.name>, Thomas Gleixner <tglx@...utronix.de>, 
    Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>, 
    Alexandre Ghiti <alexghiti@...osinc.com>, linux-riscv@...ts.infradead.org, 
    linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev
Subject: Re: [PATCH 1/4] riscv: entry: Convert ret_from_fork() to C

On Wed, 22 Jan 2025, Charlie Jenkins wrote:

> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 33a5a9f2a0d4e1eeccfb3621b9e518b88e1b0704..9225c322279aa90e737b1d7144db084319cf8103 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -319,17 +319,14 @@ SYM_CODE_END(handle_kernel_stack_overflow)
>  ASM_NOKPROBE(handle_kernel_stack_overflow)
>  #endif
>  
> -SYM_CODE_START(ret_from_fork)
> +SYM_CODE_START(ret_from_fork_asm)
>  	call schedule_tail
> -	beqz s0, 1f	/* not from kernel thread */
> -	/* Call fn(arg) */
> -	move a0, s1
> -	jalr s0
> -1:
> -	move a0, sp /* pt_regs */
> -	call syscall_exit_to_user_mode
> +	move a0, s1 /* fn */
> +	move a1, s0 /* fn_arg */
> +	move a2, sp /* pt_regs */
> +	call ret_from_fork
>  	j ret_from_exception

 Wouldn't it make sense to save a jump here and make it a tail call, i.e.:

	la ra, ret_from_exception
	tail ret_from_fork

?

  Maciej