On Wed, 2005-06-29 at 00:26 -0400, Peter Arremann wrote:
> Unfortunately AMD gets punished for Intel's lazyness. Intel does not want to
> implement an iommu.
Actually, it was more like the fact that because AMD doesn't have all
processors connect to a "hub" over the same connection, AMD was _forced_
to develop GARTs for _all_ I/O in the so-called "32-bit" Athlon MP.
The I/O MMU in the Athlon 64 and Opteron is just an evolution of that.
GARTs and I/O MMU's are _nothing_ new in RISC platforms that are
switched or meshed. Intel just hasn't developed such a beast, and 3rd
party proprietary Xeon or Itanium systems that do use "glue" (and costly
system redesign) when they need to. AMD was just the first to offer a
commodity one that comes built-in (along with other goodies).
The "bonus" is that an I/O MMU solves the _real_world_ issue that some
I/O cards and drivers only do 32-bit addressing, and are incapable of
handling memory mapped I/O above 4GiB.
> RedHat and others don't want to have to support two separate kernels - so
> they limit IO to the lowest 4GB no matter if you're running an Opteron
> or EM64T.
? I was unaware this is how they handled Opteron. I thought Red Hat
_dynamically_ handled EM64T separately in their x86-64 kernels, and that
was a major performance hit.
I need to go research this ...
> I assume this quote is from http://lwn.net/Articles/117783/? about the 4th
> page table level?
> The memory that your process can use is split in several different segments as
> listed in that article. The processes need to have (among other stuff) access
> to the kernel, shared memory and so on. For that they have to select a
> mapping - and the mapping was simply selected to support 46 bits...
Actually, it sounds like they were good with 3-level at 39-bit for the
current generation of x86-64, which only does 40-bit/1TiB. Unless, of
course, that was a compatibility issue with running 32-bit, PAE36 and
PAE52 program simultaneously.
I wonder if the 4-level is a performance hit, which is not ideal. Maybe
there is a way to disable it if there is no compatibility issue?
--
Bryan J. Smith b.j.smith@xxxxxxxx
---------------------------------------------------------------------
It is mathematically impossible for someone who makes more than you
to be anything but richer than you. Any tax rate that penalizes them
will also penalize you similarly (to those below you, and then below
them). Linear algebra, let alone differential calculus or even ele-
mentary concepts of limits, is mutually exclusive with US journalism.
So forget even attempting to explain how tax cuts work. ;->
