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Date: Fri, 5 Apr 2013 17:21:15 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@linaro.org>
To: Rob Herring <robherring2@gmail.com>
cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
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Subject: Re: [PATCH v6 4/4] xen/arm: introduce xen_early_init, use PSCI on
 xen
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On Fri, 5 Apr 2013, Rob Herring wrote:

> On 04/05/2013 02:36 PM, Nicolas Pitre wrote:
> > On Fri, 5 Apr 2013, Stefano Stabellini wrote:
> > 
> >> This is what happens:
> >>
> >> - No Xen
> >> Xen is not running on the platform and a Xen hypervisor node is not
> >> available on device tree.
> >> Everything keeps working seamlessly, this patch doesn't change anything.
> >>
> >> - we are running on Xen
> >> Xen is running on the platform, we are running as a guest on Xen and an
> >> hypervisor node is available on device tree.
> >> Let's also assume that there aren't any "arm,cci" compatible nodes on
> >> device tree because Xen wouldn't export this kind of information to any
> >> guests right now. Therefore PSCI should be used to boot secondary cpus.
> >> Because the versatile express machine sets smp_init to
> >> vexpress_smp_init_ops, vexpress_smp_init_ops will be called.
> >> vexpress_smp_init_ops sets smp_ops to vexpress_smp_ops, that *break*
> >> Xen.
> > 
> > OK I see.
> > 
> >> With this patch, xen_smp_init will be called instead of
> >> vexpress_smp_init_ops, and smp_ops will be set to psci_smp_ops,
> >> therefore *unbreaking* Xen.
> > 
> > However that breaks MCPM.
> 
> You mean on bare metal, right? For the bare metal, "xen,xen" property
> would not be present and xen_smp_init is not used. So the vexpress MCPM
> ops will be used. Aren't Dom0 cpu's basically virtual cpus? If Xen ever
> needs the MCPM support, the Xen hook itself can figure out whether to
> use MCPM support.

Well, if Xen has its own mdesc distinct from the VExpress one then 
things
are indeed fine.


Nicolas