Re: [PATCH v3 4/4] membarrier: riscv: Provide core serializing command

From: Andrea Parri <parri.andrea@gmail.com>
To: Stefan O'Rear <sorear@fastmail.com>
Cc: paul.walmsley@sifive.com, Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	mathieu.desnoyers@efficios.com, paulmck@kernel.org,
	Jonathan Corbet <corbet@lwn.net>,
	mmaas@google.com, Hans Boehm <hboehm@google.com>,
	striker@us.ibm.com, charlie@rivosinc.com, rehn@rivosinc.com,
	linux-riscv@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 4/4] membarrier: riscv: Provide core serializing command
Date: Wed, 10 Jan 2024 23:34:32 +0100	[thread overview]
Message-ID: <ZZ8beGSPHBOXl2Sq@andrea> (raw)
In-Reply-To: <add243df-47ea-4956-99c5-7bcf19dd2679@app.fastmail.com>

Hi Stefan,

> "core serialization" is a meaningless sequence of words for RISC-V users,

The expression is inherited from MEMBARRIER(2).  Quoting from the RFC
discussion (cf. [3] in the cover letter),

  "RISC-V does not have "core serializing instructions", meaning
  that there is no occurence of such a term in the RISC-V ISA. The
  discussion and git history about the SYNC_CORE command suggested
  the implementation below: a FENCE.I instruction [...]"

> The feature seems useful, but it should document what it does using
> terminology actually used in the RISC-V specifications.

In _current RISC-V parlance, it's pretty clear: we are doing FENCE.I.
As Palmer and others mentioned in the RFC, there're proposals for ISA
extensions aiming to "replace" FENCE.I, but those are still WIP. (*)

  Andrea

(*) https://github.com/riscv/riscv-j-extension