linux-kernel - Re: [Question] Alignment requirement for readX() and writeX()

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Message-ID: <CAK8P3a0w09Ga_OXAqhA0JcgR-LBc32a296dZhpTyPDwVSgaNkw@mail.gmail.com>
Date:   Fri, 30 Jul 2021 18:58:30 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Gary Guo <gary@...yguo.net>, Hector Martin <marcan@...can.st>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [Question] Alignment requirement for readX() and writeX()

On Fri, Jul 30, 2021 at 6:43 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> Hi,
>
> The background is that I'm reviewing Wedson's PR on IoMem for
> Rust-for-Linux project:
>
>         https://github.com/Rust-for-Linux/linux/pull/462
>
> readX() and writeX() are used to provide Rust code to read/write IO
> memory. And I want to find whether we need to check the alignment of the
> pointer. I wonder whether the addresses passed to readX() and writeX()
> need to be aligned to the size of the accesses (e.g. the parameter of
> readl() has to be a 4-byte aligned pointer).
>
> The only related information I get so far is the following quote in
> Documentation/driver-io/device-io.rst:
>
>         On many platforms, I/O accesses must be aligned with respect to
>         the access size; failure to do so will result in an exception or
>         unpredictable results.
>
> Does it mean all readX() and writeX() need to use aligned addresses?
> Or the alignment requirement is arch-dependent, i.e. if the architecture
> supports and has enabled misalignment load and store, no alignment
> requirement on readX() and writeX(), otherwise still need to use aligned
> addresses.
>
> I know different archs have their own alignment requirement on memory
> accesses, just want to make sure the requirement of the readX() and
> writeX() APIs.

I am not aware of any driver that requires unaligned access on __iomem
pointers, and since it definitely doesn't work on most architectures, I think
having an unconditional alignment check makes sense.

What would the alignment check look like? Is there a way to annotate
a pointer that is 'void __iomem *' in C as having a minimum alignment
when it gets passed into a function that uses readl()/writel() on it?

       Arnd