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

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Message-ID: <ZZ8beGSPHBOXl2Sq@andrea>
Date: Wed, 10 Jan 2024 23:34:32 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: Stefan O'Rear <sorear@...tmail.com>
Cc: paul.walmsley@...ive.com, Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, mathieu.desnoyers@...icios.com,
	paulmck@...nel.org, Jonathan Corbet <corbet@....net>,
	mmaas@...gle.com, Hans Boehm <hboehm@...gle.com>,
	striker@...ibm.com, charlie@...osinc.com, rehn@...osinc.com,
	linux-riscv@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] membarrier: riscv: Provide core serializing
 command

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