Memory inconsistency when mmap()ping

[Christoph M.]

On Wed, Jan 2, 2013 at 11:21 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Wed, Jan 02, 2013 at 10:59:55AM +0100, Christoph M. wrote:
>> I'm experiencing a strange problem when mmap()ping memory
>> to userspace on ARM-based Linux System.
>
> You're running into the standard problems with virtually tagged caches.

On bootup the kernel reports this:
[    0.000000] CPU: VIPT nonaliasing data cache, VIPT nonaliasing instruction c
ache

So the cache should be physically tagged and I would expect that there are
no aliasing problems.

>> Now to the technical details:
>>  * The memory is allocated using kzalloc(PAGE_SIZE, GFP_KERNEL);
>>  * The event counter is a u32 value which is stored at offset 0 of
>>    the buffer.
>>  * When accessing (read/write) the buffer from kernelspace a call
>>    to mb() is done afterwards.
>>  * My mmap() code does this:
>>      vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>
> And doing that will violate the requirements of the ARMv7 architecture
> that all mappings of the same physical memory space will be of the same
> type (let alone, have the same attributes.)

Is such a mapping possible for sharing a physical page between
userspace and kernelspace?
If yes, how is such a mapping, using the same attributes, done?

> The kernel mapping will still be 'memory like' and 'cacheable', maybe
> even with write-allocation.

Does this mean, that using uncached memory for my buffer (and use
an uncached mapping for userspace) should work?
If yes, can you point me in the right direction, to get uncached memory
from the kernel?

>From what I understand now is, that my buffer is mapped twice:
 * using caches in kernelspace and
 * uncached in userspace
This not allowed for the ARMv7 architecture and causes the problems
I encountered.
So I need a solution to either map the page either cached or uncached
on both sides (userspace and kernelspace).
Is this correct?

BR
Christoph