[PATCH v6 1/4] arm: introduce psci_smp_ops

[Stefano Stabellini]

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.

Under Xen, smp_init won't have a chance to return any value because it
is going to crash first.