lkml.org 
[lkml]   [2000]   [Aug]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectHow does the kernel map physical to virtual addresses?
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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 12:38    [W:0.051 / U:4.148 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site