Xen users - Apr 2007 - RE: xm vcpu-set command has no effect on fully virtualized machine

Hi,

I tried the command 
xm vcpu-set <domain ID> < vcpu-count> 
in Red Hat 5, to reduce the number of virtual CPU''s allotted to a
virtual
machine. I am encountering a problem in this. 
When I install a guest OS using Paravirtualization option the above command
works fine. 

However when I execute the above command for a guest OS which is fully
virtualized the command has no effect. Is this expected? Or is it a bug?   

When I type the command xm list, for a par virtualized OS, it shows the
changes that I made, whereas for a fully virtualized OS, there is no change
in the number of VCPU''s.   

Also xm mem-set <domain ID> < memory> does not work in a Fully
Virtualized
Guest OS. 


Regards,
Ramya.S.G.


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> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com 
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of 
> Sangati, Ramya Govinda Reddy
> Sent: 24 April 2007 08:15
> To: xen-users@lists.xensource.com
> Subject: [Xen-users] RE: xm vcpu-set command has no effect on 
> fully virtualized machine
> 
> Hi,
> 
> I tried the command 
> xm vcpu-set <domain ID> < vcpu-count> 
> in Red Hat 5, to reduce the number of virtual CPU''s allotted 
> to a virtual
> machine. I am encountering a problem in this. 
> When I install a guest OS using Paravirtualization option the 
> above command
> works fine. 
> 
> However when I execute the above command for a guest OS which is fully
> virtualized the command has no effect. Is this expected? Or 
> is it a bug?
Well, I guess we should say it''s a "feature" as in
"that''s not a bug,
it''s a feature". Now for that "real" answer: The reason
that this
doesn''t work is that there''s no REAL (standardized) way to
tell an OS
that there''s suddenly a different number of processors. If the a system
supports CPU hotplug at all, it will rely heavily on firmware (BIOS or
whatever) as well as special hardware in the system, and probably also
special drivers for the hotplug functinality for any particular OS that
supports CPU hotplug. 
> 
> When I type the command xm list, for a par virtualized OS, it 
> shows the
> changes that I made, whereas for a fully virtualized OS, 
> there is no change
> in the number of VCPU''s.   
> 
> Also xm mem-set <domain ID> < memory> does not work in a 
> Fully Virtualized
> Guest OS. 
Yes, the number of VCPU or bytes of memory owned by guest is maintained
by the hypervisor itself. The problem lies in telling the  OS that it''s
just gained/lost some of that. 

--
Mats
> 
> 
> Regards,
> Ramya.S.G.
> 


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> > However when I execute the above command for a guest OS which is fully
> > virtualized the command has no effect. Is this expected? Or
> > is it a bug?
>
> Well, I guess we should say it''s a "feature" as in
"that''s not a bug,
> it''s a feature". Now for that "real" answer: The
reason that this
> doesn''t work is that there''s no REAL (standardized) way
to tell an OS
> that there''s suddenly a different number of processors. If the a
system
> supports CPU hotplug at all, it will rely heavily on firmware (BIOS or
> whatever) as well as special hardware in the system, and probably also
> special drivers for the hotplug functinality for any particular OS that
> supports CPU hotplug.
Mmmmm.  Maybe this could one day be sort-of tackled using some PV drivers in 
the guest or by emulating a hardware platform that has hotpluggable CPUs 
(although I wouldn''t be surprised if there''s not yet a
satisfactory way of
doing this such that common OSes will all recognise it).

In the meantime, HVM guests need to be given the right number of CPUs at boot 
time, as Mats has said.
> > When I type the command xm list, for a par virtualized OS, it
> > shows the
> > changes that I made, whereas for a fully virtualized OS,
> > there is no change
> > in the number of VCPU''s.
> >
> > Also xm mem-set <domain ID> < memory> does not work in a
> > Fully Virtualized
> > Guest OS.
>
> Yes, the number of VCPU or bytes of memory owned by guest is maintained
> by the hypervisor itself. The problem lies in telling the  OS that
it''s
> just gained/lost some of that.
I believe 3.0.5 incorporates a balloon driver for HVM guests so that they can 
resize their memory dynamically at request if the driver has been installed 
on them.  This will be available for Linux but I expect that you''d need
to
use XenEnterprise / XenServer / XenExpress to get it for Windows.  Novell and 
Intel are planning on releasing some PV drivers for Windows guests in the 
next few months - maybe these will include balloon support.

Cheers,
Mark


-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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