linux-kernel - Re: [PATCH v6 4/4] xen/arm: introduce xen_early

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Message-ID: <alpine.DEB.2.02.1304060015150.5353@kaball.uk.xensource.com>
Date:	Sat, 6 Apr 2013 00:15:45 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Rob Herring <robherring2@...il.com>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"will.deacon@....com" <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v6 4/4] xen/arm: introduce xen_early_init, use PSCI on
 xen

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.

That is correct.


> 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.

Right.
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