Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <1618995255-91499-1-git-send-email-guoren@kernel.org>
 <20210428031807.GA27619@roeck-us.net> <CAJF2gTTSMC947zisNs+j_2rMoBqoOy-j1jvVBk2DNrf0Xt6sWA@mail.gmail.com>
 <CAK8P3a1DvsXSEDoovLk11hzNHyJi7vqNoToU+n5aFi2viZO_Uw@mail.gmail.com> <20210428124946.GA1976154@infradead.org>
In-Reply-To: <20210428124946.GA1976154@infradead.org>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 28 Apr 2021 15:48:29 +0200
Message-ID: <CAK8P3a0zHhEjfEjHYx7UJuxnG1BfMgOhBi2sWrQxV0WwwAvBWQ@mail.gmail.com>
Subject: Re: [PATCH] csky: uaccess.h: Coding convention with asm generic
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Guo Ren <guoren@kernel.org>, Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-csky@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
        Guo Ren <guoren@linux.alibaba.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Apr 28, 2021 at 2:49 PM Christoph Hellwig <hch@infradead.org> wrote:
> On Wed, Apr 28, 2021 at 11:25:29AM +0200, Arnd Bergmann wrote:
> > We might want to come up with a new version of asm-generic/uaccess.h
> > that actually makes it easier to have a sane per-architecture
> > implementation of the low-level accessors without set_fs().
> >
> > I've added Christoph to Cc here, he probably has some ideas
> > on where we should be heading.
>
> I think asm-generic/uaccess.h pretty much only makes sense for
> nommu.  For that case we can just kill the __{get,put}_user_fn
> indirection.  I actually have work for that in an old branch.
>
> Trying to use any of asm-generic/uaccess.h for MMU based kernel is
> just asking for trouble.

The one thing I'd like to see is a generic implementation of the outer
bit that implements handling the variable-length arguments, there are
so many ways that people have gotten that wrong in the past, and
it would be nice if architectures only had to implement a set of fixed-size
accessors that contain the architecture specific inline asm.

There is now a new version for x86 that based on asm-goto with
output, which should in theory provide a better implementation
for any architecture when using gcc-11/clang-11 or higher.

> > One noteworthy aspect is that almost nothing users the low-level
> > __get_user()/__put_user() helpers any more outside of architecture
> > specific code, so we may not need to have separate versions
> > for much longer.
>
> Al has been trying to kill them off entirely for a while, and I hope
> he'll eventually succeed.  That being said the difference should be
> that the __ versions just skip the access_ok, so having both is
> fairly trivial to implement.

That is the difference in the interface, but in some of architectures
there is another difference in that the __ version is completely
inlined while the normal version calls an external function.
I could never quite figure out the reason for this difference.

      Arnd