Hi Ed,
> > Why does it convert the physical address provided by pmd_val to a
> > virtual one? The addresses returned from pgd_offset and pmd_offset
> > represent a physical address, correct?
>
> I don't think so.
But the whole traversing starts with pgd_offset:
#define pgd_offset(mm, address) ((mm)->pgd+pgd_index(address))
Isn't mm_struct's pgd a pointer to a *physical* address? Since pgd's value
is used in the cr3 register I thought you had to be a *physical* address...
> In memory.c:free_one_pmd there's an example of how pmd_page is used.
> It yields a pointer to the struct page in the mem_map that corresponds
> to the page frame where the pmd is stored. That pointer is a virtual
> address.
>
> On lowmem x86, for kernel data, the relationship between a virtual
> address and a physical address is just the addition or subtraction of
> 0xC0000000. The __va macro just adds 0xC0000000 to the physical
> address.
One of the big issues I'm having is knowing when the code is using physical,
virtual, or linear address spaces.
Question: When in kernel mode, can you refer to a value/address by a
physical address *or* a virtual address and have it magically work???? If
you're running kernel code and you try to access a virtual address n, does
the MMU magically know to subtract PAGE_OFFSET from n? Otherwise, trying to
access virtual address n would result in a memory addressing error if you
had < 3 GB of physical RAM?
Please forgive my complete ignorance ;) Hopefully I'm just missing something
obvious.
Thanks,
Kirk
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