linux-kernel - Re: [PATCH v2 3/6] riscv: ftrace: prepare ftrace for atomic code patching

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Message-ID: <87wmkk8toq.fsf@all.your.base.are.belong.to.us>
Date: Tue, 13 Aug 2024 14:59:33 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Andy Chiu <andy.chiu@...ive.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
 <aou@...s.berkeley.edu>, Alexandre Ghiti <alexghiti@...osinc.com>, Zong Li
 <zong.li@...ive.com>, Steven Rostedt <rostedt@...dmis.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers
 <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Justin Stitt
 <justinstitt@...gle.com>, Puranjay Mohan <puranjay@...nel.org>
Cc: Palmer Dabbelt <palmer@...osinc.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, Andy Chiu <andy.chiu@...ive.com>
Subject: Re: [PATCH v2 3/6] riscv: ftrace: prepare ftrace for atomic code
 patching

Andy Chiu <andy.chiu@...ive.com> writes:

> We use an AUIPC+JALR pair to jump into a ftrace trampoline. Since
> instruction fetch can break down to 4 byte at a time, it is impossible
> to update two instructions without a race. In order to mitigate it, we
> initialize the patchable entry to AUIPC + NOP4. Then, the run-time code
> patching can change NOP4 to JALR to eable/disable ftrcae from a
                                      enable        ftrace

> function. This limits the reach of each ftrace entry to +-2KB displacing
> from ftrace_caller.
>
> Starting from the trampoline, we add a level of indirection for it to
> reach ftrace caller target. Now, it loads the target address from a
> memory location, then perform the jump. This enable the kernel to update
> the target atomically.

The +-2K limit is for direct calls, right?

...and this I would say breaks DIRECT_CALLS (which should be implemented
using call_ops later)?


Björn