Re: [PATCH edk2-platforms v2 1/1] Silicon/SynQuacer: add optional OP-TEE DT node

[Sumit Garg <sumit.garg@linaro.org>]

On Fri, 27 Jul 2018 at 18:19, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Thu, Jul 26, 2018 at 02:12:04PM +0530, Sumit Garg wrote:
> > On Thu, 26 Jul 2018 at 13:20, Daniel Thompson <daniel.thompson@linaro.org> wrote:
> > > I guess it could implement a secure monitor call to provide it. In
> > > fact I find it a rather pleasing approach. However I think it still loops
> > > us round to pretty much the same question as before. Does TF-A "protec
> > > " a normal world that makes an SMC to an OP-TEE that isn't there by
> > > failing the call in a nice way?
> >
> > TF-A returns SMC call for OP-TEE as unknown (error code: -1 in "x0"
> > register) if OP-TEE is not present.
>
> Be careful here; you can't use an arbitrary SMC since that could be
> implemented by another trusted OS (with a completely different meaning).
>
> Assuming you know the system provides SMCCC, you can use the "Call UID
> Query" in the trusted OS range, and check that returned value matches
> OP-TEE's UID.
>
> i.e
>
>   uid = smccc_uid_query(OPTEE_RANGE);
>   if (uid == OPTEEE_SMCCC_UID) {
>           [ OP-TEE present ]
>   } else {
>           [ unknown/no trusted OS present ]
>   }
>

Thanks Mark for this useful suggestion. Will try to use it.

-Sumit

> Thanks,
> Mark.