On Oct 17, 2012 2:22 PM, "Kshemendra KP" <kshemendra@xxxxxxxxxxxxx> wrote:
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> On Wed, Oct 17, 2012 at 2:04 PM, Chetan Nanda <chetannanda@xxxxxxxxx> wrote:
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>> On Tue, Oct 16, 2012 at 5:47 PM, Kshemendra KP <kshemendra@xxxxxxxxxxxxx> wrote:
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>>> On x86 kernel is normally split into 3GB (user) : 1 GB (Kernel) spaces. Kernel can only directly
>>> manipulate 1 GB (around 889 MB) from the PAGE_OFFSET (0xC0000000). The user space
>>> memofy below PAGE_OFFSET kernel can't directly access, it considers this memory as
>>> high memory. Kernel needs to kmap()/kmap_atomic() map user page and access that region.
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>> As per my understanding, if kernel code is running under a process context then it can access lower 3GB address space, provided address in 0-3GB is in process address space. And its not a high memory.
>> High memory is a virtual address space with 1GB kernel space to map RAM pages beyond 896MB.
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> One thing not clear to me is, during boot, kernel is copied to 1st MB of the RAM in x86 architecture as this architecture
> has ISA mem map hole (640k -to 1MB). From 1st BM till 896MB is occupied by the kernel. Then user space will be
> made available beyond 896 MB. With this reasoning, it is not clear to me, whether user space which is present
> beyond 896 MB is in high memory.
On x86 kernel will be loaded in RAM from 1st MB. But kernel code and data will not go upto 896MB.
Kernel code + data will be loaded in ram from 1MB - end_data (IIRC). and address space between end_data to 896Mb is used for dynamic allocation (kmalloc ) etc...
I would recomend you to read 2nd chapter of understanding linux kernel. This chapter contain detailed info on memory addressing used by kernel on x86
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>>> If the memory is above 4 GB with page extension, it is clear that kernel can't access it as
>>> kernel uses "void *" and/or "unsigned long" to hold the address. But it is not clear for me
>>> why kernel's can't directly access memory below PAGE_OFFSET ( 0-3GB) directly.
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>> Kernel can access 0-3GB memory, take an example of application sending data to kernel via system call e.g. write.
>> in Write system call pointer to the buffer will be in 0-3Gb address space of calling application. And kernel will access this address to write data to drivers/etc ..
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>>> Regards
>>>
>>> Kshemendra
>>>
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>>>
Chetan Nanda
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