Changing the page attributes

[Arun KS]

On Wed, Jul 11, 2012 at 12:04 PM, Arun KS <getarunks at gmail.com> wrote:
> Hello Mulyadi,
>
> On Tue, Jul 10, 2012 at 9:42 PM, Mulyadi Santosa
> <mulyadi.santosa at gmail.com> wrote:
>> Hi..
>>
>> On Tue, Jul 10, 2012 at 6:51 PM, Arun KS <getarunks at gmail.com> wrote:
>>> Hello Experts,
>>>
>>> I have a requirement to change __log_buf memory(kernel printk buffer)
>>> as non cacheable memory.
>>> How can I change the page attributes for changing cache policy to non cacheable?
>>
>> I just can give short clue. I think somehow you must find its PTE
>> entry...there, IIRC there is page attribute that can be changed to
>> make it uncacheable.
>>
>> cross check the Intel processor manual for further clarification (if
>> it's indeed x86 arch)
>
> __log_buf size is 512KB and PTE is mapped to a 1MB section. So if I
> change the attribute for this 1MB section, lot of other stuffs(in this
> 1MB bss section) will also become non-cachable. Looks like it is not
> possible unless my __log_buf is 1MB in size.
> Or I need to create another section like .init, .data, .bss and create
> 4KB pages(with two levels of Page Tables) instead of 1M section.
>
> My  arch is arm-v7

I went ahead and made my __log_buf size to 1MB.
But the memory is not aligned to 1MB boundary. Here is what system.map shows.

c08ac400 b __warned.32427
c08ac404 b __log_buf
c09ac404 B console_set_on_cmdline
c09ac408 b idle_notifier
c09ac40c B sys_tz

so __log_buf resides in two 1M sections in parts and it needs two PTEs.

Then I added __aligned__(1024*1024) attribute to __log_buf array.
This time compiler error  "Error: alignment too large: 15 assumed"

But  __aligned_(32*1024) is working fine as below

c08b84c0 b __warned.32427
c08c0000 b __log_buf
c09c0000 B console_set_on_cmdline
c09c8000 b idle_notifier
c09c8004 B sys_tz

Is it not possible to align more than 32KB?
Any pointers?

Thanks,
Arun

>
> Thanks,
> Arun
>>
>> --
>> regards,
>>
>> Mulyadi Santosa
>> Freelance Linux trainer and consultant
>>
>> blog: the-hydra.blogspot.com
>> training: mulyaditraining.blogspot.com