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Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and
 improve documentation
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     x86 <x86@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        stable <stable@vger.kernel.org>
References: <cover.1623813516.git.luto@kernel.org>
 <07a8b963002cb955b7516e61bad19514a3acaa82.1623813516.git.luto@kernel.org>
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From:   Andy Lutomirski <luto@kernel.org>
Message-ID: <678def50-eecd-fb0f-eac0-c0ba1a442574@kernel.org>
Date:   Thu, 17 Jun 2021 17:13:24 -0700
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On 6/17/21 8:16 AM, Mathieu Desnoyers wrote:
> ----- On Jun 15, 2021, at 11:21 PM, Andy Lutomirski luto@kernel.org wrote:
> 
> [...]
> 
>> +# An architecture that wants to support
>> +# MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE needs to define precisely what it
>> +# is supposed to do and implement membarrier_sync_core_before_usermode() to
>> +# make it do that.  Then it can select ARCH_HAS_MEMBARRIER_SYNC_CORE via
>> +# Kconfig.Unfortunately, MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is not a
>> +# fantastic API and may not make sense on all architectures.  Once an
>> +# architecture meets these requirements,
> 
> Can we please remove the editorial comment about the quality of the membarrier
> sync-core's API ?

Done
>> +#
>> +# On x86, a program can safely modify code, issue
>> +# MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE, and then execute that code, via
>> +# the modified address or an alias, from any thread in the calling process.
>> +#
>> +# On arm64, a program can modify code, flush the icache as needed, and issue
>> +# MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE to force a "context synchronizing
>> +# event", aka pipeline flush on all CPUs that might run the calling process.
>> +# Then the program can execute the modified code as long as it is executed
>> +# from an address consistent with the icache flush and the CPU's cache type.
>> +#
>> +# On powerpc, a program can use MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
>> +# similarly to arm64.  It would be nice if the powerpc maintainers could
>> +# add a more clear explanantion.
> 
> We should document the requirements on ARMv7 as well.

Done.

> 
> Thanks,
> 
> Mathieu
>