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Re: [Xen-devel] [PATCH] add dom0 vcpu hotplug control

To: Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] add dom0 vcpu hotplug control
From: Ryan Harper <ryanh@xxxxxxxxxx>
Date: Tue, 7 Jun 2005 17:35:14 -0500
Cc: Ryan Harper <ryanh@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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* Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx> [2005-06-07 17:27]:
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> > Ryan Harper
> > Sent: 07 June 2005 23:03
> > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: [Xen-devel] [PATCH] add dom0 vcpu hotplug control
> > 
> > This patch adds new control messages for vcpu hotplug events. 
> >  Via the xm vcpu_hotplug sub-program, VCPUS in domains can be 
> > enabled/disabled when CONFIG_HOTPLUG_CPU is enabled in the 
> > target domain's kernel.
> > 
> > Currently there is nothing that tracks whether a VCPU is up or down.
> > My previous [1]patch added a new per-VCPU flag (VCPUF_down) 
> > which could be used to keep track of which VCPUS are up and 
> > down.  Right now, there isn't a hypercall that returns the 
> > per-VCPU flag status (something equivalent to the per-domain 
> > flags in get_dominfo ).  Have we thought about a 
> > get_vcpu_info hypercall?  I'd like to get that implemented so 
> > I can report VCPU state in the xm list --vcpus output.  That 
> > would also make it easier to skip sending control messages 
> > that don't change the VCPU's state (e.g. sending a down 
> > message to a vcpu that is already down).  
> I haven't looked at the code, but can you not use DOM0_GETVCPUCONTEXT to
> iterate over all the VCPUs for a domain? (e.g. if you ask for VCPU N and
> it doesn't exist (isn't up) you'll get the state for the next highest
> numbered one, or ESRCH if there isn't one) That's the behaviour I'd
> expect.

That would work.  Moving forward, there may be more per-VCPU info that
we want to collect and that it would make sense to factor out per-VCPU
info from DOM0_GETDOMINFO and create a DOM0_GETVCPUINFO call.  If that
is not desirable at this point, I can work with DOM0_GETVCPUCONTEXT.

Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
(512) 838-9253   T/L: 678-9253

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