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

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Message-ID: <YQQ3KAXrPN1CuglL@boqun-archlinux>
Date:   Sat, 31 Jul 2021 01:30:16 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
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>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [Question] Alignment requirement for readX() and writeX()

On Fri, Jul 30, 2021 at 06:58:30PM +0200, Arnd Bergmann wrote:
> 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?
> 

If we want to check, I'd expect we do the checks inside
readX()/writeX(), for example, readl() could be implemented as:

	#define readl(c) 					\
	({							\
		u32 __v;					\
								\
		/* alignment checking */			\
		BUG_ON(c & (sizeof(__v) - 1));			\
		__v = readl_relaxed(c);				\
		__iormb(__v);					\
		__v;						\
	})

It's a runtime check, so if anyone hates it I can understand ;-)

Regards,
Boqun

>        Arnd