How does the kernel map physical to virtual addresses?

From: Timur Tabi (
Date: Fri Aug 25 2000 - 18:26:25 EST

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 -
Interactive Silicon -

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