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SubjectRe: Linux address mapping
   Date: 	Fri, 4 Jun 1999 15:45:54 +0200 (MET DST)
From: Bjorn Wesen <bjorn@sparta.lu.se>

Where in the memory space is this placed, and are there any
particular restrictions/requirements for placing this ?

Look for the VMALLOC* macros in include/asm-*/*.h
Another thing I was thinking about, is if the kernel is mapped into every
process page table, is this done on a page-by-page basis, i.e a 600 kb
kernel is mapped using for example 600/4 PTE's linearily, but maybe
processes can share the second level page table for the kernel mapping so
each process don't need its own kernel PTE map set.

It depends upon the architecture, some mmus do it right enough that
the kernel is transparently mapped correctly for each process.

I wonder if there is an "ideal" MMU for Linux, i.e. one that maximizes
performance and minimizes the porting work :)

My heavily biased opinion is that UltraSparc is the current mainstream
"closest to ideal". The user and the kernel happily live in the same
virtual addresses, and are seperated by address space identifiers.
It's like a segment on x86, but imho much cleaner. This setup means
that the user can have the entire (32-bit or 64-bit) virtual address
space, if the whole thing is available in the hardware (most
current generation 64-bit cpus do not, and have a "hole" in the middle
of the 64-bit virtual address space which is not usable, Alpha,
UltraSparc-I, and UltraSparc-II fall into this category). But from
what is available, the user can map all of it on UltraSparc.

As an example, from the kernel we can access user address space only
when we set the correct attributes in a load or store instruction, so:

ldx [user_address] ASI_AIUS, %reg

Loads an 8-byte word, at 'user_address', from the ASI_AIUS (stands for
"Address Space Indentifier, As If from User Secondary") address space,
into %reg.

This means it uses the "secondary user" translations in the MMU (the
UltraSparc has three address spaces, primary, secondary, and nucleus,
the first two have settable "MMU context" values, whereas the nucleus
is fixed at "MMU context" zero which is where the kernel thus lives).
And it accesses the data as if it were done in non-privileged mode,
thus full protection checking are done as if the user did it himself.

During user process execution, the "primary" context is set to
the appropriate MMU context value for his address space. At trap time
the cpu automatically enters the nucleus context as the default
(ie. the one which normal loads and stores, without attributes, will
use) The trap entry code saves the critical register state etc. and
then sets the "primary" context to zero (ie. nucleus) and sets the
"secondary" context to what the user had.

This is how it all works out.

A nice side effect of this is that kernel page tables are explicit in
every access, and since UltraSparc uses software filled TLBs the Linux
kernel can manage the kernel page tables seperately, and in any way it
likes (and thus a minimally and efficiently as it likes) since we are
not fixed into any hardware imposed page table structure. For
example, most of the kernel address space can have it's mapping
computed in constant time because of their linear physical-->virtual
nature, we take advantage of this by not having any page table state
for it at all on UltraSparc.

Alpha works similarly, except:

1) without the primary/secondary contexts
2) "software managed" TLBs are filled by PAL code

Up until now the Alpha port has not modified the PAL code from the
reference version provided by Digital, so in essence the page table
structure used by Tru64/DUX/whatever is what Linux is using. There is
no reason this couldn't be changed and experimented with in the future
though.

Later,
David S. Miller
davem@redhat.com


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