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[Xen-devel] Re: [RFC PATCH 00/35] Xen i386 paravirtualization support

To: Chris Wright <chrisw@xxxxxxxxxxxx>
Subject: [Xen-devel] Re: [RFC PATCH 00/35] Xen i386 paravirtualization support
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Wed, 22 Mar 2006 11:15:40 -0600
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Chris Wright wrote:
Xen also provides support for running directly on native hardware.

Can someone elaborate on this? Does this mean a Xen guest can run on bare metal?

Is there code available to make this work (it doesn't seem contained in this patchset)? Has any performance analysis been done?

The numbers that have been posted with the VMI patches suggest that some rather tricky stuff is required to achieve native performance when running a guest on bare metal. If this is not the case, it would be very interesting to know because it seems to be the hairiest part of the VMI patches.

Otherwise, if we want to support Xen guests on bare metal, it seems we would have to change things in the subarch code a bit to do something similar to VMI.


Anthony Liguori

The following patch series provides the minimal support required to
launch Xen paravirtual guests on standard x86 hardware running the Xen
hypervisor.  These patches effectively port the Linux kernel to run on the
platform interface provided by Xen.  This port is done as an i386 subarch.

With these patches you will be able to launch an unprivileged guest
running the modified Linux kernel and unmodified userspace.  This guest
is x86, UP only, runs in shadow translated mode, and has no direct access
to hardware.  This simplifies the patchset to the minimum functionality
needed to support a paravirtualized guest.  It's worth noting that
a fair amount of this patchset deals with paravirtualizing I/O, not
just CPU-only.  The additional missing functionality is primarily about
full SMP support, optimizations such as direct writable page tables,
and the management interface.  Those refinements will be posted later.

At a high-level, the patches provide the following:

- Kconfig and Makefile changes required to support Xen
- subarch changes to allow more platform functionality to be
  implemented by an i386 subarch
- Xen subarch implementation
- start of day code for running in the hypervisor provided environment (paging
- basic Xen drivers to provide a fully functional guest

The Xen platform API encapsulates the following types of requirements:

- idt, gdt, ldt (descriptor table handling)
- cr2, fpu_taskswitch, debug registers (privileged register handling)
- mmu (page table, tlb, and cache handling)
- memory reservations
- time and timer
- vcpu (init, up/down vcpu)
- schedule (processor yield, shutdown, etc)
- event channel (generalized virtual interrupt handling)
- grant table (shared memory interface for high speed interdomain communication)
- block device I/O
- network device I/O
- console device I/O
- Xen feature map
- Xen version info

Thanks to those who provided early feedback to this patchset: Andi Kleen,
Gerd Hoffmann, Jan Beulich, Rik van Riel, Stephen Tweedie, and the Xen team.
And thanks to the Xen community: AMD, HP, IBM, Intel, Novell, Red Hat,
Virtual Iron, XenSource -- see Xen changelog for full details.

Known issues:

        CodingStyle conformance is still in progress
        /proc interface needs to be replaced with something more appropriate
        entry.S cleanups are possible


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