[PATCH RFC] PPC: /dev/mem: All RAM is cacheable, not just the kernel's.

[Li Yang]

On Tue, Oct 19, 2010 at 6:32 AM, Scott Wood <scottwood at freescale.com> wrote:
> If mem= is used on the kernel command line to create reserved regions
> for userspace to map using /dev/mem, let it be mapped cacheable as long
> as it is within the memory region described in the device tree.
>
> Signed-off-by: Scott Wood <scottwood at freescale.com>
> ---
> This isn't just a performance issue, but it could also be a correctness
> issue, if the reserved portion of RAM is mapped cacheable by e.g. a KVM
> guest.  This patch does not address cases where such regions could show up
> as something other than a standard memory node -- such as shared regions
> in an AMP configuration.  Ideally there would be some means for a platform
> to register cacheable regions, without having to completely replace the
> entire phys_mem_access_prot function.

Agreed.  Such requirement might be special case in server/desktop
world, but much more common in embedded market when migrating to
multi-core in which shared memory is commonly used for inter-core
communication.  It will be best to have a common way to achieve this.

Not only cache-ability but also coherent property might need to be tunable.

- Leo