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

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Date:	Fri, 5 Apr 2013 20:15:51 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
cc:	Rob Herring <robherring2@...il.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 Sat, 6 Apr 2013, Stefano Stabellini wrote:

> On Fri, 5 Apr 2013, Nicolas Pitre wrote:
> > 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.
> 
> It's not about the mdesc: Xen has its own hypervisor node on device tree
> if and only if Xen is running on the platform, therefore the Xen early
> hook is never going to do anything at all on native.
> 
> In other words, this patch should NOT change the behaviour of Linux on
> native, and if it did, it would be a bug.

Perfect.


Nicolas
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