Basic HighMeM Question

[piyush moghe]

Thanks Prabhu.

So does this means that this ZONE_NORMAL limit of 896MB is because of 3:1
memory division?

If it is do then does this mean that if this ratio is changed then
ZONE_NORMAL limit will also be changed?


Regards,
Piyush

On Mon, Jun 27, 2011 at 4:45 PM, Prabhu nath <gprabhunath at gmail.com> wrote:

>
> On Mon, Jun 27, 2011 at 3:58 PM, Paraneetharan Chandrasekaran <
> paraneetharanc at gmail.com> wrote:
>
>>
>>
>> On 27 June 2011 14:58, Prabhu nath <gprabhunath at gmail.com> wrote:
>>
>>> Please see inline.
>>>
>>> On Mon, Jun 27, 2011 at 2:42 PM, piyush moghe <pmkernel at gmail.com>wrote:
>>>
>>>> I have very basic some question's related to HighMem Memory Mapping:
>>>>
>>>> 1) Why can't we directly map memory in highmemory?
>>>>
>>>         This question is incorrect ? Typically on a intel architecture,
>>> the physical address space is divided into LOWMEM and HIGHMEM region. Lower
>>> 896 MB is marked as LOWMEM region and >896MB as HIGHMEM. More likely in the
>>> intel architecture the memory is always decoded from 0x00000000. For Eg. If
>>> you have memory of 1GB, then 896MB is decoded to LOWMEM and the rest is
>>> decoded in HIGHMEM.
>>>
>>>>
>>>> 2) As documents at many places why is the limit of 896MB for
>>>> ZONE_NORMAL?
>>>>
>>>         Since this 896 MB of physical address space is directly mapped to
>>> the Kernel linear virtual address space i.e from (0xC0000000 to 0xF8000000).
>>> Also, there is a Fixed constant offset relation between VA to PA i.e. VA =
>>> PA + 0xC000000. This has been done to avoid any page table walk for
>>> translating kernel virtual address to physical address.
>>> Thus when kernel virtual address is generated for execution, PA is
>>> calculated by MMU directly, thus making kernel code execution faster.
>>>
>>
>> Does this mean MMU doesnt look into TLB or pagetable when the kernel
>> virtual address is referenced? how does the MMU know the range of direct
>> mapping ( i.e offset-ed mapping)?
>>
>        Ideally yes, MMU should hold the range information. I do not know
> about Intel architecture but I have learnt that in PowerPC architecture
> there is a BAT register which will hold the offset information.
>       Any comments ?
>
>>
>>> All the above explaination strictly holds good for Intel architecture on
>>> Desktop machines.
>>>
>>> Regards,
>>> Prabhunath
>>>
>>>>
>>>>
>>>> Regards,
>>>> Piyush
>>>>
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>>>>
>>>
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>>>
>>
>>
>> --
>> Regards,
>> Paraneetharan C
>>
>
>
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