linux-kernel - Re: [PATCH v4 12/12] riscv: Documentation: add a description about dynamic ftrace

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Message-ID: <47286334f978e1a9e19efa6fd3af3d06de863080.camel@rivosinc.com>
Date: Fri, 11 Apr 2025 14:02:30 +0200
From: Robbin Ehn <rehn@...osinc.com>
To: Andy Chiu <andybnac@...il.com>, linux-riscv@...ts.infradead.org, 
	alexghiti@...osinc.com, palmer@...belt.com
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, Alexandre Ghiti
	 <alex@...ti.fr>, bjorn@...osinc.com, puranjay12@...il.com, 
	paul.walmsley@...ive.com, greentime.hu@...ive.com, nick.hu@...ive.com, 
	nylon.chen@...ive.com, eric.lin@...ive.com, vicent.chen@...ive.com, 
	zong.li@...ive.com, yongxuan.wang@...ive.com, samuel.holland@...ive.com, 
	olivia.chu@...ive.com, c2232430@...il.com
Subject: Re: [PATCH v4 12/12] riscv: Documentation: add a description about
 dynamic ftrace

Hi, thanks,

s/atmoic/atomic

On Tue, 2025-04-08 at 02:08 +0800, Andy Chiu wrote:
> +without a race condition. This series makes atmoic code patching possible in
...
> +setting the first instruction to AUIPC, and the second to NOP. Now, atmoic

/Robbin

> +patching is possible because the kernel only has to update one instruction.
> +According to Ziccif, as long as an instruction is naturally aligned, the ISA
> +guarantee an  atomic update.
> +
> +By fixing down the first instruction, AUIPC, the range of the ftrace
> trampoline
> +is limited to +-2K from the predetermined target, ftrace_caller, due to the
> lack
> +of immediate encoding space in RISC-V. To address the issue, we introduce
> +CALL_OPS, where an 8B naturally align metadata is added in front of each
> +pacthable function. The metadata is resolved at the first trampoline, then
> the
> +execution can be derect to another custom trampoline.
> +
> +CMODX in the User Space
> +---------------------
> +
> +Though fence.i is an unprivileged instruction, the default Linux ABI
> prohibits
> +the use of fence.i in userspace applications. At any point the scheduler may
> +migrate a task onto a new hart. If migration occurs after the userspace
> +synchronized the icache and instruction storage with fence.i, the icache on
> the
> +new hart will no longer be clean. This is due to the behavior of fence.i only
> +affecting the hart that it is called on. Thus, the hart that the task has
> been
> +migrated to may not have synchronized instruction storage and icache.
>  
>  There are two ways to solve this problem: use the riscv_flush_icache()
> syscall,
>  or use the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` prctl() and emit fence.i in