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

Boqun Feng
2021-07-30T17:30:48Z

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@gmail.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