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

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Message-ID: <e4e1ee4d-b96c-47c7-bf81-5f4d3b0ce5dc@efficios.com>
Date: Wed, 24 Jan 2024 13:56:39 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
 paulmck@...nel.org, corbet@....net, mmaas@...gle.com, 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

On 2024-01-24 13:44, Andrea Parri wrote:
>>> +# riscv uses xRET as return from interrupt and to return to user-space.
>>> +#
>>> +# Given that xRET is not core serializing, we rely on FENCE.I for providing
>>> +# core serialization:
>>> +#
>>> +#  - by calling sync_core_before_usermode() on return from interrupt (cf.
>>> +#    ipi_sync_core()),
>>> +#
>>> +#  - via switch_mm() and sync_core_before_usermode() (respectively, for
>>> +#    uthread->uthread and kthread->uthread transitions) to go back to
>>> +#    user-space.
>>
>> I don't quite get the meaning of the sentence above. There seems to be a
>> missing marker before "to go back".
> 
> Let's see.  Without the round brackets, the last part becomes:
> 
>    - via switch_mm() and sync_core_before_usermode() to go back to
>      user-space.
> 
> This is indeed what I meant to say.  What am I missing?

Would it still fit your intent if we say "before returning to
user-space" rather than "to go back to user-space" ?

Because the switch_mm(), for instance, does not happen exactly on
return to user-space, but rather when the scheduler switches tasks.
Therefore, I think that stating that core serialization needs to
happen before returning to user-space is clearer than stating that
it happens "when" we go back to user-space.

Also, on another topic, did you find a way forward with respect of
the different choice of words between the membarrier man page and
documentation vs the RISC-V official semantic with respect to "core
serializing" vs FENCE.I ?

Thanks,

Mathieu


> 
>    Andrea

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com