linux-kernel - Re: [PATCH v6 1/4] arm: introduce psci_smp

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Date:	Thu, 18 Apr 2013 13:38:32 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"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@....com" <will.deacon@....com>,
	"arnd@...db.de" <arnd@...db.de>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"marc.zyngier@....com" <marc.zyngier@....com>
Subject: Re: [PATCH v6 1/4] arm: introduce psci_smp_ops

On Thu, 18 Apr 2013, Stefano Stabellini wrote:

> On Thu, 18 Apr 2013, Nicolas Pitre wrote:
> > On Thu, 18 Apr 2013, Stefano Stabellini wrote:
> > 
> > > On Thu, 18 Apr 2013, Russell King - ARM Linux wrote:
> > > > This should allow the Xen problem to be resolved, because Xen will
> > > > provide the PSCI operations, and it's correct in that case to override
> > > > the platform's SMP operations.
> > > 
> > > Yes, increasing the priority of PSCI helps Xen a lot.
> > > In order to completely solve the issue for Xen though, another patch is
> > > needed (http://marc.info/?l=linux-kernel&m=136630106201968&w=2) because
> > > of the introduction of smp_init.
> > 
> > Please look at the latest smp_init patch version I sent to you.  It 
> > shouldn't conflict with Xen any longer.  It now returns a bool result 
> > depending on whether it did set up smp_ops or not.
> 
> CPUs are virtualized by Xen and do not reflect or expose the underlying
> SMP hardware and firmware features, so an hardware specific smp_init
> cannot run.
> 
> So the smp_init patch still breaks Xen because even if smp_init can fail
> graciously, executing a platform specific smp_init function that tries
> to access registers and memory regions that are not present is going to
> cause an undefined behaviour.

It won't access hardware but just look into the DT and return false if 
nothing interesting is found.  At which point the next attempt in the 
priority list is PSCI by default.


Nicolas
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