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

[Catalin Marinas <catalin.marinas@xxxxxxx>]

On 4 December 2012 18:14, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Tue, Dec 04, 2012 at 06:02:13PM +0000, Nicolas Pitre wrote:
>> On Tue, 4 Dec 2012, Will Deacon wrote:
>> > 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.

We can have enable-method DT entries independent of the SoC and one of
them can be psci-hvc.

Just for clarification, AArch32 with virtualisation mandates the
security extensions, so the SMC can be trapped. On AArch64 it is a bit
tricky since the presence of EL3 is not mandate, in which case SMC
would undef (don't as why ;). That's where we can have different
enable methods specified via the DT.

-- 
Catalin

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