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

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Date:	Wed, 3 Apr 2013 10:00:02 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
cc:	"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, 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?


Nicolas
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