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RE: [Xen-users] Status of frequency scaling

  • To: "Stephan Seitz" <nur-ab-sal@xxxxxx>, "xen-users" <xen-users@xxxxxxxxxxxxxxxxxxx>
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Mon, 30 Apr 2007 11:47:03 +0200
  • Delivery-date: Mon, 30 Apr 2007 02:46:01 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: AceJ02UChP6ex4t0RUmYzAhYRr5CqwBOAMKQ
  • Thread-topic: [Xen-users] Status of frequency scaling


> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Stephan Seitz
> Sent: 28 April 2007 21:25
> To: xen-users
> Subject: [Xen-users] Status of frequency scaling
> Hi!
> I would like to know the status of the frequency scaling in 
> XEN. I found 
> a web site saying that XEN doesn't work with frequency 
> scaling, at the 
> other hand I found some fixes for powernow-k8 to get rid of some 
> messages.

Well, the current official status is "it doesn't work at all". 

There are several problems, some of which are relatively easy to fix and
some that aren't:
1. The white-list of MSR's that can be written doesn't include the
FID/VID MSR's. This is VERY easy to fix - a few lines of code added to
traps.c in the function dealing with MSR read/write operations will take
care of that. 
2. There is no power-management policy for the entire system, so dom0
usage will determine the entire system's performance. This doesn't work
very well, as Dom0 isn't supposed to be heavily loaded, DomU's are. 
3. Guest may need to be informed of the frequency change. Some kernels
may not appreciate that the CPU frequency is chaning "without it's
knowledge". [This is probably not a big problem on modern OS's, but some
older ones may be very un-co-operative on this account]. 

> I have an AMD64. powerno-k8 works find with non-XEN kernels, but with 
> XEN-kernels I get the error:
> powernow-k8: Found 1 AMD Athlon(tm) 64 Processor 3400+ 
> processors (version 2.00.00)
> powernow-k8: BIOS error - no PSB or ACPI _PSS objects

This is strange - as far as I can remember, I had powernow working on my
machine not so long ago (3.0.4 or so) with only the traps.c patched to
allow writing of the FID/VID controls registers. 

You're doing this in Dom0 I take it? I don't expect this to work in


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