linux-kernel - Re: [PATCH] arm64: Add the arm64.nolse

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Message-ID: <84f0994a-26de-c20a-a32f-ec8fe41df3a3@quicinc.com>
Date:   Tue, 11 Jul 2023 18:15:49 +0800
From:   "Aiqun(Maria) Yu" <quic_aiquny@...cinc.com>
To:     Will Deacon <will@...nel.org>
CC:     <corbet@....net>, <catalin.marinas@....com>, <maz@...nel.org>,
        <quic_pkondeti@...cinc.com>, <quic_kaushalk@...cinc.com>,
        <quic_satyap@...cinc.com>, <quic_shashim@...cinc.com>,
        <quic_songxue@...cinc.com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

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

-- 
Thx and BRs,
Aiqun(Maria) Yu