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[Xen-ia64-devel] Time for hybrid virtualization?

To: xen-ia64-devel <xen-ia64-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-ia64-devel] Time for hybrid virtualization?
From: Alex Williamson <alex.williamson@xxxxxx>
Date: Wed, 16 Jan 2008 17:02:07 -0700
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Hi all,

   I've been running some simple kernel compile benchmarks lately and
getting much better HVM results than I was expecting.  In summary, both
PV and HVM performance are within 11-12% of native performance for an
SMP guest build on a Montvale system.  For the same test on x64, PV
shows ~12% overhead while HVM is ~55%.  This begs the question; is
paravirtualziation on ia64 worthwhile?

   Jun Nakajima has presented his paper[1] on hybrid virtualization at a
few conferences and at one point asked me what I thought about running
Dom0 in VT mode.  At the time, I didn't have good VT-i performance data
and assumed it was possible, but had little advantage.  However, if
we're only looking at a very small performance gap between PV and HVM
already, perhaps we should give this some serious consideration.

   This idea becomes especially interesting now as we're thinking about
how to get Xen/ia64 support into upstream kernels.  If we require VT
processors, could we significantly reduce the complexity and
intrusiveness of the Linux kernel changes required for Dom0/DomU
support?  In the best case, I could imagine that if we can rely on VT,
then xen-ification of the Linux kernel might be as "simple" as creating
a Xen aware machine vector (different from the one we have now) and a
set of hypercall support interfaces.  I would envision Dom0 operating
similar to a PCI pass-through domain with direct access to the hardware
(but not requiring VT-d).  We would obviously need the PV drivers to
become more pervasive to avoid the I/O bottleneck that exists with Qemu
and regain PV DomU-like I/O performance for guests.

   Is it possible?  Is it a good idea?  What are some of the issues?  We
would lose support for non-VT capable processors (pre-Montecito), but is
that so bad?  Is it a "fast track" to upstream Linux Xen/ia64 support?
Let me know your thoughts.  Thanks,


[1] http://ols.108.redhat.com/2007/Reprints/nakajima-Reprint.pdf

Alex Williamson                             HP Open Source & Linux Org.

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