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

Aiqun(Maria) Yu
2023-07-11T10:55:50Z

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.

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.
>
>>> Please can you explain the problem that you are trying to solve?
>>
>> In our current case, it is a 100% reproducible issue that happened for
>> uncached data, the cpu which support LSE atomic, but the system's DDR
>> subsystem is not support this and caused a NOC error and thus synchronous
>> external abort happened.
>
> So? The Arm ARM allows this behaviour and Linux shouldn't run into it.
>
> Will