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

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Message-ID: <ZMxDe0gXKYbY5jgt@andrea>
Date:   Fri, 4 Aug 2023 02:16:59 +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

> Can you double-check that riscv switch_mm() implies a fence.i or equivalent
> on the CPU doing the switch_mm ?

AFAICT, (riscv) switch_mm() does not guarantee that.

> AFAIR membarrier use of sync_core_before_usermode relies on switch_mm
> issuing a core serializing instruction.

I see.  Thanks for the clarification.

BTW, the comment in __schedule() suggests that membarrier also relies on
switch_mm() issuing a full memory barrier: I don't think this holds.

Removing the "deferred icache flush" logic in switch_mm() - in favour of
a "plain" MB; FENCE.I - would meet both of these requirements.

Other ideas?

  Andrea