Re: How does the kernel map physical to virtual addresses?

[Tigran Aivazian <tigran@veritas.com>]

Hi,

it is interesting to observe that many questions that deal with _details_
are answered quickly but questions related to fundamental concepts related
to how Linux is designed, baffle all of us (since 0 people answered). So,
is there really nobody in the whole world who can answer this? I would
like to know the answer (about global kernel memory layout - i.e. what
goes into PSE pages and what goes into normal ones, and how does PAE mode
change the picture?) myself...

I suppose one could find the answer by looking at each element of
mem_map[] but it is always more comfortable to look for answers when one
already knows the correct answer beforehand.

Regards,
Tigran


 On Fri, 25 Aug 2000, Timur Tabi wrote:

> When my driver wants to map virtual to physical (and vice versa) addresses, it
> calls virt_to_phys and phys_to_virt. All these macros do is add or subtract a
> constant (PAGE_OFFSET) to one address to get the other address.
> 
> How does the kernel configure the CPU (x86) to use this mapping?  I was under
> the impression that the kernel creates a series of 4MB pages, using the x86's
> 4MB page feature.  For example, in a 64MB machine, there would be 16 PTEs (PGDs?
> PMDs?), each one mapping a consecutive 4MB block of physical memory.  Is this
> correct?  Somehow I believe that this is overly simplistic.
> 
> The reason I ask is that I'm confused as to what happens when a user process or
> tries to allocate memory.  I assume that the VM uses 4KB pages for this
> allocatation.  So do we end up with two virtual addresses pointing the same
> physical memory?  
> 
> What happens if I use ioremap_nocache() on normal memory?  Is that memory
> cached or uncached?  If I use the pointer obtained via phys_to_virt(), the
> memory is cached.  But if I use the pointer returned from ioremap_nocache(), the
> memory is uncached.  My understanding of x86 is that caching is based on
> physical, not virtual addresses.  If so, it's not possible for a physical
> address to be both cached and uncached at the same.
> 
> Could someone please straighten me out?
> 
> 
> 
> --
> Timur Tabi - ttabi@interactivesi.com
> Interactive Silicon - http://www.interactivesi.com
> 
> When replying to a mailing-list message, please don't cc: me, because then I'll just get two copies of the same message.
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