Re: [PATCH -next v2 2/2] riscv: extable: add new extable type EX_TYPE_KACCESS_ERR_ZERO support

From: Tong Tiangen <tongtiangen@huawei.com>
To: Andrew Jones <ajones@ventanamicro.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Palmer Dabbelt <palmer@rivosinc.com>,
	Albert Ou <aou@eecs.berkeley.edu>, <Conor.Dooley@microchip.com>,
	<linux-riscv@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
	<wangkefeng.wang@huawei.com>, Guohanjun <guohanjun@huawei.com>
Subject: Re: [PATCH -next v2 2/2] riscv: extable: add new extable type EX_TYPE_KACCESS_ERR_ZERO support
Date: Sat, 27 Aug 2022 18:39:38 +0800	[thread overview]
Message-ID: <301cd27e-d003-584b-2eb0-8fa025348cff@huawei.com> (raw)
In-Reply-To: <20220826081607.ci42sbffrmsfhk2w@kamzik>

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.

Thanks,
Tong.

> 
> .