[PATCH v4] arm: Fix memory attribute inconsistencies when using fixmap

[afzal.mohd.ma@gmail.com (afzal mohammed)]

Hi,

On Thu, Apr 06, 2017 at 05:03:14PM +0100, Jon Medhurst (Tixy) wrote:
> To cope with the variety in ARM architectures and configurations, the
> pagetable attributes for kernel memory are generated at runtime to match
> the system the kernel finds itself on. This calculated value is stored
> in pgprot_kernel.
> 
> However, when early fixmap support was added for ARM (commit
> a5f4c561b3b1) the attributes used for mappings were hard coded because
> pgprot_kernel is not set up early enough. Unfortunately, when fixmap is
> used after early boot this means the memory being mapped can have
> different attributes to existing mappings, potentially leading to
> unpredictable behaviour. A specific problem also exists due to the hard
> coded values not include the 'shareable' attribute which means on
> systems where this matters (e.g. those with multiple CPU clusters) the
> cache contents for a memory location can become inconsistent between
> CPUs.
> 
> To resolve these issues we change fixmap to use the same memory
> attributes (from pgprot_kernel) that the rest of the kernel uses. To
> enable this we need to refactor the initialisation code so
> build_mem_type_table() is called early enough. Note, that relies on early
> param parsing for memory type overrides passed via the kernel command
> line, so we need to make sure this call is still after
> parse_early_params().

Tested-by: afzal mohammed <afzal.mohd.ma@gmail.com>

with an emphasis on no-MMU's

Regards
afzal