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

["Grzegorz Miłoś" <gm281@xxxxxxxxx>]

> 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.

> 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

Hope this helps.

cheers
Gr(z)egor(z)

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