Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

Aiqun(Maria) Yu
2023-07-12T03:09:56Z

On 7/11/2023 6:25 PM, Will Deacon wrote:
> On Tue, Jul 11, 2023 at 06:15:49PM +0800, Aiqun(Maria) Yu wrote:
>> On 7/11/2023 4:22 PM, Will Deacon wrote:
>>> On Tue, Jul 11, 2023 at 12:02:22PM +0800, Aiqun(Maria) Yu wrote:
>>>> On 7/10/2023 5:37 PM, Will Deacon wrote:
>>>>> On Mon, Jul 10, 2023 at 01:59:55PM +0800, Maria Yu wrote:
>>>>>> In order to be able to disable lse_atomic even if cpu
>>>>>> support it, most likely because of memory controller
>>>>>> cannot deal with the lse atomic instructions, use a
>>>>>> new idreg override to deal with it.
>>>>>
>>>>> This should not be a problem for cacheable memory though, right?
>>>>>
>>>>> Given that Linux does not issue atomic operations to non-cacheable mappings,
>>>>> I'm struggling to see why there's a problem here.
>>>>
>>>> The lse atomic operation can be issued on non-cacheable mappings as well.
>>>> Even if it is cached data, with different CPUECTLR_EL1 setting, it can also
>>>> do far lse atomic operations.
>>>
>>> Please can you point me to the place in the kernel sources where this
>>> happens? The architecture doesn't guarantee that atomics to non-cacheable
>>> mappings will work, see "B2.2.6 Possible implementation restrictions on
>>> using atomic instructions". Linux, therefore, doesn't issue atomics
>>> to non-cacheable memory.
>>
>> We encounter the issue on third party kernel modules and third party apps
>> instead of linux kernel itself.
>
> Great, so there's nothing to do in the kernel then!
>
> The third party code needs to be modified not to use atomic instructions
> with non-cacheable mappings. No need to involve us with that.

>
>> This is a tradeoff of performance and stability. Per my understanding,
>> options can be used to enable the lse_atomic to have the most performance
>> cared system, and disable the lse_atomic by stability cared most system.
>
> Where do livelock and starvation fit in with "stability"? Disabling LSE
> atomics for things like qspinlock and the scheduler just because of some
> badly written third-party code isn't much of a tradeoff.
We also have requirement to have cpus/system fully support lse atomic
and cpus/system not fully support lse atomic with a generic kernel image.
Same kernel module wanted to be used by lse atomic fully support cpu and
not fully support cpu/system as well.

That's why we want to have a runtime option here.

>
> Will