Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation

Andy Lutomirski
2021-06-18T00:14:51Z

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