memcpy alignment

Jon Masters jcm at
Tue Dec 15 08:28:56 PST 2015

On 12/15/2015 11:09 AM, Leif Lindholm wrote:
> 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.

Yeah. That was the reason I didn't just change the existing initrd code
in the first place (wanted to leave it as is). I *did not know* memcpy
to/from Device memory was explicitly banned (and I get why, and I do
know one is supposed to map Device memory as such, etc. etc.) for this
reason. However it's actually very reasonable to demand correctness
going in. If it were hacked/kludged to paper over the situation I
describe it would stand even less chance of being fixed.

I would /separately/ note that there's an inefficiency in that the
existing code relies upon assumed equal alignment between src/dst so the
hardware is probably doing a lot of silent unaligned writes.


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