memcpy alignment

[Mark Rutland]

On Tue, Dec 15, 2015 at 10:43:03AM -0500, Jon Masters wrote:
> On 12/15/2015 10:32 AM, Russell King - ARM Linux wrote:
> > On Tue, Dec 15, 2015 at 10:24:13AM -0500, Jon Masters wrote:
> >> On 12/15/2015 04:34 AM, Catalin Marinas wrote:
> >>> On Mon, Dec 14, 2015 at 11:11:02PM -0500, Jon Masters wrote:
> >>>> What's supposed to happen if the natural alignment of the pointers dst
> >>>> and src is not the same? We don't seem to handle that case today, and
> >>>> instead will base our access data type on the source alignment only.
> >>>
> >>> The hardware takes care of the other one. The memcpy behaviour in the
> >>> kernel is the same as the glibc one (the Cortex Strings library).
> >>
> >> Not if you're dealing with Device memory. I accept that one could always
> >> ensure that there's a non-Device mapping (I got the other replies) but I
> >> don't think it's unreasonable to expect a memcpy to work in the presence
> >> of misaligned addresses either.
> > 
> > Using memcpy() on memory returned from functions which setup IO/MMIO
> > mappings has always been outlawed.  It's the whole reason we have
> > things like memcpy_toio(), memcpy_fromio() and memset_io(), which
> > date back a few decades now, and they exist for exactly these kinds
> > of reasons.
> > 
> > If you get an __iomem pointer, then you must respect that it
> > essentially can not be non-dereferenced, and you must use one of the
> > standard kernel accessors (read[bwl]/ioread*/write[bwl]/iowrite*/
> > memcpy_fromio/memcpy_toio/memset_io) to access it.  That's the API
> > contract you implicitly signed up to by using something like ioremap()
> > or other mapping which gives you an iomem mapping.
> 
> Thanks Russell. If it's definitely never allowed then the existing x86
> code needs to be fixed to use an IO access function in that case. I get
> that those accessors are there for this reason, but I wanted to make
> sure that we don't ever expect to touch Device memory any other way (for
> example, conflicting mappings between a VM and hypervisor). I am certain
> there's other non-ACPI code that is going to have this happen :)

It's important to note that much of the ACPI code was written prior to
the availability of memremap.

Depending on the case, the better solution may be to use memremap. I
would expect this to be the case for any static tables of information,
at least.

Thanks,
Mark.