Re: [PATCH v6 1/4] arm: introduce psci_smp_ops

From: Nicolas Pitre <nicolas.pitre@linaro.org>
To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
	"xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	"arnd@arndb.de" <arnd@arndb.de>,
	"rob.herring@calxeda.com" <rob.herring@calxeda.com>,
	"marc.zyngier@arm.com" <marc.zyngier@arm.com>
Subject: Re: [PATCH v6 1/4] arm: introduce psci_smp_ops
Date: Fri, 19 Apr 2013 11:47:27 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LFD.2.03.1304191146230.17375@syhkavp.arg> (raw)
In-Reply-To: <alpine.DEB.2.02.1304191035360.7254@kaball.uk.xensource.com>

On Fri, 19 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, 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.
> 
> OK, that should work.
> I am going to drop "xen/arm: introduce xen_early_init, use PSCI on xen".
> 
> We'll have to be careful enforcing that future smp_init implementations
> use DT rather than probing registers. Maybe I should add a comment to
> the smp_init patch to clarify that?

No one should be probing registers without making sure it is safe to do 
so.  Even on non virtualized hardware this can be a dangerous thing to 
do.

Nicolas