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

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Message-ID: <515F392B.1030200@gmail.com>
Date:	Fri, 05 Apr 2013 15:50:51 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	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 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.

Rob

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