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

[Nicolas Pitre <nico@xxxxxxxxxxx>]

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?

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.

> 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.

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


Nicolas

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