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Date: Fri, 27 Jul 2018 13:49:12 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Sumit Garg <sumit.garg@linaro.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>, edk2-devel@lists.01.org,
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Subject: Re: [PATCH edk2-platforms v2 1/1] Silicon/SynQuacer: add optional OP-TEE DT node
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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.