linux-kernel - Re: [PATCH -next v2 2/2] riscv: extable: add new extable type EX_TYPE_KACCESS_ERR

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Date:   Wed, 21 Sep 2022 13:25:04 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...osinc.com>
To:     tongtiangen@...wei.com
CC:     ajones@...tanamicro.com, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, Conor.Dooley@...rochip.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        wangkefeng.wang@...wei.com, guohanjun@...wei.com
Subject:     Re: [PATCH -next v2 2/2] riscv: extable: add new extable type EX_TYPE_KACCESS_ERR_ZERO support

On Sat, 27 Aug 2022 03:39:38 PDT (-0700), tongtiangen@...wei.com wrote:
>
>
> 在 2022/8/26 16:16, Andrew Jones 写道:
>> On Fri, Aug 26, 2022 at 02:44:48PM +0800, Tong Tiangen wrote:
>>>
>>>
>>> 在 2022/8/25 19:06, Andrew Jones 写道:
>>>> On Mon, Aug 15, 2022 at 03:20:25AM +0000, Tong Tiangen wrote:
>>>>> Currently, The extable type EX_TYPE_UACCESS_ERR_ZERO is used by
>>>>> __get/put_kernel_nofault(), but those helpers are not uaccess type, so we
>>>>> add a new extable type EX_TYPE_KACCESS_ERR_ZERO which can be used by
>>>>> __get/put_kernel_no_fault().
>>>>>
>>>>> Only refactor code without any functional changes.
>>>>
>>>> This isn't quite true. __get/put_kernel_nofault now sets a different
>>>> extable type (as the commit message says). But, nothing special seems
>>>> to be done with that, so there's effectively no functional change. Can
>>>> you please elaborate on the motivation for this change? Where will the
>>>> KACCESS type need to be distinguished from the UACCESS type?
>>>
>>> The introduction of EX_TYPE_KACCESS_ERR_ZERO does not change any function,
>>> but makes a correct distinction in the actual type, indicating that there
>>> are indeed some kaccess entries in extable. I think this optimization is
>>> more clear and reasonable.
>>
>> Well, creating new types, just for new type sake, just bloats code.
>>
>>>
>>> A few weeks ago, I did something similar on arm64[1]. I think this
>>> optimization can also be used on riscv.
>>>
>>> We can do some features that are used on uaccss but not applicable on
>>> kaccess in the future[2].
>>>
>>> [1]
>>> https://lore.kernel.org/lkml/20220621072638.1273594-2-tongtiangen@huawei.com/
>>> [2]https://lore.kernel.org/lkml/20220812070557.1028499-4-tongtiangen@huawei.com/
>>>
>>
>> This is part of the information, but I had already found this. What's
>> still missing to me are the riscv patches, or at least a riscv plan, for
>> actually implementing something which requires kaccess and uaccess to have
>> distinct types.
>>
>> Thanks,
>> drew
>
> At present, there is no such plan on riscv, because it is rely on
> hardware support.
> I think this patch can be merged as a small code optimization and
> without any function change.

Generally we need some use of the code in the upstream kernel to justify 
its existence.  In this case I don't really see that: it's just another 
type that's exactly the same as the existing one, having some out of 
tree code that depends on making these types do something different 
isn't a sufficient justification.