memcpy alignment

[Leif Lindholm]

On Tue, Dec 15, 2015 at 10:43:03AM -0500, Jon Masters wrote:
> > 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 :)

A lot of code that has never run on anything other than x86 will have
such issues.

Tracking the use of page_is_ram() around the kernel, looking at what
it does for different architectures, and looking at how its (not
formalised) semantics are interpreted can also be quite unsettling.

/
    Leif