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

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Message-ID: <20220826081607.ci42sbffrmsfhk2w@kamzik>
Date:   Fri, 26 Aug 2022 10:16:07 +0200
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Tong Tiangen <tongtiangen@...wei.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Albert Ou <aou@...s.berkeley.edu>, Conor.Dooley@...rochip.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        wangkefeng.wang@...wei.com, Guohanjun <guohanjun@...wei.com>
Subject: Re: [PATCH -next v2 2/2] riscv: extable: add new extable type
 EX_TYPE_KACCESS_ERR_ZERO support

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