Re: [Xen-devel] Vcpu allocation for a newly created domU

[Sandesh <sandesh.ahiremath@xxxxxxxxx>]

On Wed, 2008-06-18 at 12:13 +0100, Grzegorz MiÅoÅ wrote:
> > Hi all,
> >
> >  I am having confusion regarding the way a newly created domain is
> > allocated vcpu.
> >  Initially during dom0 creation alloc_vcpu is called to create vcpu
> > structs for all the available cpu's and assigned to dom0. But its not
> > the case for domU creation.
> > 1. So how will dom0 relinquish/share vcpu to/with a newly created domU.
> > Does this happen as part of the shared_info page mapping??
>
> vcpus are created for each new domain, they are not shared with Dom0.
in xen/arch/x86/setup.c right after domain_create(0,0, DOM0_SSIDREF)
which creates dom0, alloc_vcpu(dom0..) is called which will allocate
vcpu for dom0. Then in construct_dom0 alloc_vcpu is called for
dom0_max_cpus. So aren't all vcpu's getting allocated to dom0??

Or is it that domU will get allocated separate vcpu's, irrespective of
these above vcpu's??
>
> > 2. Till the end of DOMCTL_createdomain hypercall d->vcpu is null but by
> > the start of xc_linux_bulid_internal function d->vcpu is pointing to
> > some vcpu. How is this??
>
> vcpus are created as a part of DOMCTL_max_vcpus. This hypercall is
> issued by the tools in Dom0, specifically by: _constructDomain() in
> python/xen/xend/XendDomainInfo.py. Look for the following line:
> xc.domain_max_vcpus(self.domid, int(self.info['VCPUs_max']))
>

> > 3. As part of the domU config file we can constraint the domU to a
> > specific CPU. How is this achieved unless there is an explict way of
> > assigning vcpu associated with that cpu number.
>
> This is done by the following call chain:
> DOMCTL_setvcpuaffinity ->
> vcpu_set_affinity ->
> __vcpu_set_affinity
>
Ohh ya!! i had completely missed the above two part...now things are
much clear.
> Hope this helps.
 Thanks for the info, it certainly cleared some amount of confusion.
> cheers
> Gr(z)egor(z)


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