Re: [Xen-devel] [PATCH 1/4] xen/arm: basic PSCI support, implement cpu_on

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-04-09 at 15:23 +0100, Marc Zyngier wrote:
> On 09/04/13 14:57, Ian Campbell wrote:
> > On Thu, 2013-03-21 at 18:42 +0000, Stefano Stabellini wrote:
> >> Implement support for ARM Power State Coordination Interface, PSCI in
> >> short. The current implementation is based on HVC and only supports the
> >> cpu_on call.
> > 
> > Doesn't the PSCI interface require the use of SMC not HVC?
> 
> You can use both, and KVM uses HVC.

It makes good sense for Xen to follow suit then I think.

> > I thought I heard Charles say at connect that there was now a PSCI v2,
> > and I suspect I'm looking at the v1 document (which mentions HVC only in
> > passing). Which interface did you implement?
> > 
> > Anyhow, we can trap SMCs to the hypervisor by setting the right control
> > register bits. We should do this anyway -- no good can come of a guest
> > making a call direct to the monitor!
> 
> Trapping guest access to Secure mode is always a good idea! ;-)

:-)

> Unfortunately, there's a catch on ARMv8. If the CPU doesn't implement
> secure mode, then SMC will UNDEF at the current exception level (not
> trapping to EL2). Which means that for ARMv8, you basically have to
> mandate HVC for PSCI at the HYP level...

That pretty much seals it then!

Do I infer that on v7 SMC w/o security extensions will trap?

Ian.


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