Re: [XenARM] [RFC PATCH 0/2] Add support for a fake, para-virtualised machine

[Will Deacon <will.deacon@xxxxxxx>]

On Tue, Dec 04, 2012 at 06:02:13PM +0000, Nicolas Pitre wrote:
> On Tue, 4 Dec 2012, Will Deacon wrote:
> 
> > Hi Nicolas,
> > 
> > On Tue, Dec 04, 2012 at 05:00:07PM +0000, Nicolas Pitre wrote:
> > > on the topic of a para-virtualised machine, I think that it should 
> > > simply implement the PSCI calls to bring up CPUs _without_ any holding 
> > > pen nor spinning tables.  You issue the appropriate PSCI call with the 
> > > physical address for secondary_startup() as argument and you're done.  
> > > The host intercepts that call and free a new CPU instance in response.  
> > > That's all.
> > 
> > I'd be happy to go with this suggestion if it wasn't for one thing:
> > platforms that do not implement a secure mode. For these platforms, smc will
> > be an undefined instruction at the exception level where it is executed and
> > therefore cannot be trapped by the hypervisor.
> 
> Really?  I thought the hypervisor could virtualize SMC calls.  Or is 
> that considered a security hazard?

If the security extensions aren't implemented, the hypervisor can't trap the
smc instruction.

> I don't remember all the PSCI spec details, but I think there was some 
> provision for this case i.e. the SMC call could be a HYP call instead.  
> And if that's not in the spec, then it probably should be added and 
> implemented as if it was.

Well, this depends on the guest taking an undefined instruction exception on
the smc, then deciding to issue an hvc instead and *then* having the
hypervisor somehow translate that into a PSCI invocation. It could work, but
it sounds easy to mess up and relies on the PSCI firmware co-existing with
things like kvm.

> > If that situation requires a pen, I see no benefit from having two boot
> > schemes where one of them would work in every case.
> 
> We always have the choice between several schemes in device drivers for 
> example, depending on the hardware generation.  Yet we always implement 
> the better scheme for the newest hardware for performance reasons, even 
> if an older one could work in all cases.

Again, I totally agree when it comes to things like poweroff and hotplug but
for booting I don't think we gain much from having multiple implementations
for a single platform. Hopefully this is moot -- see below.

> A holding pen is a rather stupid scheme.  Please let's try to do without 
> it if possible.

I've just hacked up Rob's suggestion and it seems to be working, so I'll
post a pen-less v2 tomorrow. The hotplug/reboot code can come later when we
have something host-side that we can use (could be PSCI).

Will

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