linux-kernel - Re: [PATCH v5 3/3] xen/arm: introduce xen_early

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Message-ID: <alpine.DEB.2.02.1304031525550.5078@kaball.uk.xensource.com>
Date:	Wed, 3 Apr 2013 15:47:08 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>
Subject: Re: [PATCH v5 3/3] xen/arm: introduce xen_early_init, use PSCI on
 xen

On Wed, 3 Apr 2013, Nicolas Pitre wrote:
> On Wed, 3 Apr 2013, Stefano Stabellini wrote:
> 
> > On Tue, 2 Apr 2013, Nicolas Pitre wrote:
> > > On Tue, 2 Apr 2013, Stefano Stabellini wrote:
> > > 
> > > > @@ -176,27 +178,30 @@ static int __init xen_secondary_init(unsigned int cpu)
> > > >  	return 0;
> > > >  }
> > > >  
> > > > +static void __init xen_smp_init(void)
> > > > +{
> > > > +	if (psci_smp_available())
> > > > +		smp_set_ops(&psci_smp_ops);
> > > > +}
> > > > +
> > > 
> > > I still don't understand why you need to do this.  Why can't you just 
> > > rely on the next priority which is to set PSCI ops by default if 
> > > available?  Using this hook for Xen doesn't look justified. As it is, 
> > > you break MCPM.
> > > 
> > > As I explained to you, MCPM will end up using PSCI as well when 
> > > available, which I think should be sufficient for your concern.
> > 
> > The smp_init hook is not limited to MCPM or the versatile express
> > platform. It's a generic hook that can be used by any platform and can
> > override the platform smp_ops or the psci_smp_ops depending on platform
> > specific configurations.
> > 
> > Configurations that I am pretty sure won't be available on Xen anyway,
> > while I am certain that using psci_smp_ops would work.
> > 
> > It seems to me that relying on the fact that only versatile express and
> > only MCPM use smp_init, even though it might work today, it's very
> > fragile and could break tomorrow without any of us noticing.
> 
> I claim: this breaks MCPM today.
>
> You claim: the alternative _could_ break with Xen tomorrow.
> 
> Could we please wait until there is an actual problem with Xen before 
> being overly defensive in the code?

Sorry, I realized that I should have explained myself better.

The smp_init patch together with the MCPM patch series
(http://marc.info/?l=linux-arm-kernel&m=136004188122492) breaks Xen Dom0
SMP support on versatile express *today* (ifndef CONFIG_CLUSTER_PM):
smp_init is not NULL on versatile express anymore therefore smp_init is
going to be called, causing vexpress_smp_ops to be selected even if psci
is on device tree.

On the other hand if this patch was applied, xen_smp_init would override
smp_init making sure that psci_smp_ops is used on Xen and everything
would work as expected.


So as it stands today, we need this patch for regular Xen Dom0 SMP
support.


BTW I have actually tried it on my versatile express machine to make
sure that it is correct :)
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