Re: Xen, PAE, and NetBSD

Matthias Scheler <tronzhadum.org.uk> writes:

> On Sun, Feb 11, 2007 at 12:25:10AM -0500, Todd Vierling
wrote:
>> Runs fine on a PAE mode 32-bit Linux host that you
do not have the
>> option to reconfigure?
>
> Out of technical curiosity:
> Why does a domU have to support PAE? AFAIK the page
table is managed by
> the Hypervisor. Is the interface to the Hypervisor so
badly designed
> that it can't hide such details?

The page table is managed by the domain; it's just that
writes to it
have to be checked by the hypervisor in order to maintain
security.

So, the domain maps the page table read-only, and can then
read from
it, including the referenced/modified bits maintained by the
MMU, as
efficiently as any other memory.  And, while writes do
require a
hypercall, many of them can be queued up and flushed with
one call.

The design philosophy here, as I understand it, is that the
hypervisor
should do the minimum necessary to securely and fairly
multiplex the
hardware, and that further abstraction be left to the guest
OS, which
will know best what kind of abstraction it wants.  (In fact,
it does
less than that, because I/O devices are handled by a
privileged
domain.)  It's similar to the goal of a microkernel or
exokernel
system, though details differ.

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