linux-kernel - Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

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Message-ID: <ZM0STfpkRSfNQBt8@andrea>
Date:   Fri, 4 Aug 2023 16:59:25 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     paulmck@...nel.org, paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, mmaas@...gle.com, hboehm@...gle.com,
        striker@...ibm.com
Subject: Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

> What is the relationship between FENCE.I and instruction cache flush on
> RISC-V ?

The exact nature of this relationship is implementation-dependent.  From
commentary included in the ISA portion referred to in the changelog:

  A simple implementation can flush the local instruction cache and
  the instruction pipeline when the FENCE.I is executed.  A more
  complex implementation might snoop the instruction (data) cache on
  every data (instruction) cache miss, or use an inclusive unified
  private L2 cache to invalidate lines from the primary instruction
  cache when they are being written by a local store instruction.  If
  instruction and data caches are kept coherent in this way, or if
  the memory system consists of only uncached RAMs, then just the
  fetch pipeline needs to be flushed at a FENCE.I.  [..]

Mmh, does this help?

  Andrea