Xen users - May 2006 - Running Xen on current Opteron family?

> I have been looking at XenSource 3.0 solution for running virtual
> servers on opteron servers. According to XenSource''s
documentation, it
> runs on Opterons with pacifica feature. My understanding is that we
> need this feature for following reaons:
> - lower virtualization overhead. Theoretically, 3-5% overhead for
> guest oses when virtualized
> - running guest OSes without source code - useful for running Windows
> 2003 in addition to linux and solaris for testing
> 
> However, the pacifica feature is not present in current opteron
> family. After googling, I found out that it has been renamed to
> "Secure Virtual Machine Extension" for opterons and is scheduled
for
> release for upcoming Athlon processor releases with socket
''F''. I
> checked with HP and they confirmed that pacifica feature is planned to
> be released with socket ''F'' in opterons. There is no firm
release date
> yet, to the best of my knowledge.
> 
> Given that pacifica feature is not implemented in Opterons (unless
> "Direct Connection Architecture" qualifies as a pacifica
feature?), my
> question is whether XenSource 3.0 can still be run on current opteron
> 2xx families with low overhead and without source code for guest OSes,
> as documented in the "info" section of XenSource 3.0.
> 
> Thanks,
> Sanket

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To run unmodified guests, you do need SVM (AMD Virtualization
Technology, formely codenamed Pacifiica). Nothing else will work. And by
the way, don''t expect a 3-5% overhead when running unmodified
OS''s, it''s
just not possible with that type of architecture, without further
additions, such as para-virtualized drivers. This obviously will depend
on the amount of hardware accesses that happen in the system, but it''s
most likely going to be a fair bit higher. The 3-5% figure is based on
para-virtulized guests, so guests that have modified source-code. It
would be possible to get pretty close to that by having para-virtualized
drivers, so that hardware accesses are not done by intercepting the
actual hardware access and translatiing each individual hardware access
into a packet of data-request for the service OS and then transferring
the result back to the guest with another set of intercepts - each
intercept is quite time-consuming, and a driver that had a more direct
interface (rather than using a "hardware" interface, we''d use
a
"software" interface, so the information is more conveniently packaged
for the service-OS, and there''s much less need to work out
what''s going
on from multiple hardware accesses). 
 
The current generation of Opteron processors does support
Para-virtualized Xen. There will be new models of Opteron processors
with SVM available later this year - I''m not sure exactly when, as
I''m
not part of the team that launches new processors [and of course, even
_IF_ I did know, I would not be allowed to say, as release dates are
supposed to be secret...]
 
--
Mats
 
 


________________________________

	From: xen-users-bounces@lists.xensource.com
[mailto:xen-users-bounces@lists.xensource.com] On Behalf Of Sanket
Agrawal
	Sent: 24 May 2006 16:36
	To: xen-users@lists.xensource.com
	Subject: [Xen-users] Running Xen on current Opteron family?
	
	

	I have been looking at XenSource 3.0 solution for running
virtual servers on opteron servers. According to XenSource''s
documentation, it runs on Opterons with pacifica feature. My
understanding is that we need this feature for following reaons:

	- lower virtualization overhead. Theoretically, 3-5% overhead
for guest oses when virtualized 
	- running guest OSes without source code - useful for running
Windows 2003 in addition to linux and solaris for testing 

	However, the pacifica feature is not present in current opteron
family. After googling, I found out that it has been renamed to "Secure
Virtual Machine Extension" for opterons and is scheduled for release for
upcoming Athlon processor releases with socket ''F''. I checked
with HP
and they confirmed that pacifica feature is planned to be released with
socket ''F'' in opterons. There is no firm release date yet, to
the best
of my knowledge.

	Given that pacifica feature is not implemented in Opterons
(unless "Direct Connection Architecture" qualifies as a pacifica
feature?), my question is whether XenSource 3.0 can still be run on
current opteron 2xx families with low overhead and without source code
for guest OSes, as documented in the "info" section of XenSource 3.0.

	Thanks, 
	Sanket 



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Quoting Sanket Agrawal <sanket.agrawal@appian.com>:
> > - lower virtualization overhead. Theoretically, 3-5% overhead for 
> > guest oses when virtualized 
 
AFAIK, the presence or absence of Pacifica/VT doesn''t affect the
overhead.
 
historically, virtualization systems (VMWare, QEmu, VirtualPC, etc) on x86 had 
to use heavy emulation techniques to compensate for the lack of real hardware 
virtualization. 
 
Xen uses a different tradeoff, it relies on modifications to the guest OS.  it 
doesn''t need any new processor feature, and doesn''t use it if
present.
 
of course, as we all know, that doesn''t work with closed source OSs, so
the
Xen people began to work on a real virtualization feature.  The tradeoff 
chosen was to make it depend on the new hardware features so it
wouldn''t get
much worse overhead than the already working paravirtualization. 
 
In short: 
 
without Pacifica/VT: 
 - paravirtualized guests (linux, bsd, solaris): very low overhead 
 - unmodified guests: can''t run. 
with Pacifica/VT: 
 - paravirtualized guests (linux, bsd, solaris): very low overhead (same as 
before) 
 - unmodified guests: run with somewhat bigger overhead; but not so bad as old 
software-only solutions. 
 
or, another way to see it: 
 
modifiable OS: can run paravirtualized, very low overhead.  doesn''t
need nor
use any new virtualization feature of the processor. 
non-modifiable OS: need Pacifica/VT processors, get higher overhead on disk 
and graphics. 
 
 
there are some plans about using some new CPU features even on paravirtualized 
domains to get lower cross-domain switching times, or easier PCI 
administration, or to mix 64bit/PAE/32bit guests.... but all those are in the 
future. 
 
------ 
Javier 
 
 

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