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[Xen-devel] RE: [Xen-users] Xen 3.0 Dom0 Smp enable?

To: "Ryan Harper" <ryanh@xxxxxxxxxx>
Subject: [Xen-devel] RE: [Xen-users] Xen 3.0 Dom0 Smp enable?
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Mon, 8 Aug 2005 18:04:54 +0100
Cc: Stephan Diestelhorst <sd386@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Alan Greenspan <alan@xxxxxxxxxxx>
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Thread-topic: [Xen-users] Xen 3.0 Dom0 Smp enable?
[I've moved this to xen-devel from xen-users]

> > If you create a pincpu mask for the over domains/VCPUs that doesn't 
> > include the CPU in question, they won't use it. I guess a shorthand 
> > command for this would be useful.
> Something like:
> xm dedicate DOM CPU

It should probably be called cpu-dedicate or such like.

> > Ryan, if so, please can you dust off and resend. As I 
> recall, the key 
> > fix it needed was that it should be xend (not xm) that does the 
> > translation from the dot'ed form into CPU numbers.
> Certainly it will be easier to do so in xend, but I'm more 
> concerned with the hypervisor being responsible responsible 
> for choosing the vcpu to cpu mapping.  I'd like to see dom 
> creation support the passing of a cpumap and have the 
> hypervisor cycle through the physical cpus marked therein.  
> This would remove the logic of mapping vcpus to cpus from the 
> hypervisor and let the dom creation tools build whatever 
> algorithm for distributing vcpus across cpus as it sees fit.

Sure - feel free to remove the logic from Xen. It's simply there because
until recently xend didn't know about number of hyperthreads, cores,
sockets etc.

The default should be to give Xen considerable freedom in how it
schedules VCPUs to CPUs: Stephan's load balancer is almost ready for

We should have some higher-level allocation control that can be tweaked
in xend e,g, "don't use hyperthreading", "allow only privileged domains
to use the 1st hyperthread on each CPU", "allow any logical CPU to be
used by any VCPU". [All additionally subject to cpu-dedicate

> If you are interested in this it means changing the hypercall 
> interface and so it should be done for 3.0, however I don't 
> want to push for significant changes so close to the testing 
> freeze as I want to help close features down rather than 
> create new ones.

Does it actually need a change to the hypercall interface? The domain
builder can just issue a pin for each VCPU. It might be cleaner to
remove the orignal CPU field, but that's such a small change we should
just do it anyway.

We could remove the current default allocation code from Xen.


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