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

[Daniel Thompson <daniel.thompson@linaro.org>]

On 26/07/18 09:42, Sumit Garg wrote:
> On Thu, 26 Jul 2018 at 13:20, Daniel Thompson
> <daniel.thompson@linaro.org> wrote:
>>
>> On Thu, Jul 26, 2018 at 09:39:37AM +0200, Ard Biesheuvel wrote:
>>> On 26 July 2018 at 09:36, Daniel Thompson <daniel.thompson@linaro.org> wrote:
>>>> On Wed, Jul 25, 2018 at 12:04:58PM +0200, Ard Biesheuvel wrote:
>>>>> On 23 July 2018 at 15:19, Sumit Garg <sumit.garg@linaro.org> wrote:
>>>>>> OP-TEE is optional on Developerbox controlled via SCP firmware. To check
>>>>>> if we need to delete OP-TEE DT node, we use DRAM1 region info as SCP
>>>>>> firmware conditionally carves out Secure memory from DRAM1 region.
>>>>>>
>>>>>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>>>>> Cc: Leif Lindholm <leif.lindholm@linaro.org>
>>>>>> Contributed-under: TianoCore Contribution Agreement 1.1
>>>>>> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
>>>>>> ---
>>>>>>
>>>>>
>>>>> As discussed on IRC, i am not a fan of inferring the presence of
>>>>> OP-TEE from the base/size values of the first DRAM region.
>>>>>
>>>>> Please refer to the existing PCIe code how to read a GPIO in PEI and
>>>>> set a dynamic PCD accordingly, so you can use its value in
>>>>> PlatformDxe.
>>>>
>>>> For Trusted Firmware I asked Sumit to look for the OP-TEE memory carve
>>>> out rather than looking at the switches. This was based on concerns
>>>> about version skew (new C-A53 firmware, old SCP firmware[1]), in particular
>>>> if TF-A jumps to an OP-TEE that isn't actually loaded the system will
>>>> fail in a not very transparent way (especially if the user hasn't found
>>>> the debug UART behind the back panel yet).
>>>>
>>>> What is the consequence of passing a DT with OP-TEE present if one is
>>>> not actually present? Do we at least get as far as bringing up the
>>>> framebuffer before things explode?
>>>>
> 
> If we pass a DT with OP-TEE and OP-TEE not present, Linux TEE generic
> driver exits gracefully giving following message:
> 
> [    1.976021] optee: probing for conduit method from DT.
> [    1.976033] optee: api uid mismatch

That certainly means we can be pretty relaxed about version skew of 
normal world components (since nothing bad happens if thinks get skewed).


>>> Is there any way we can let OP-TEE supply a DT overlay?
>>
>> 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.

It is possible to experiment with getting EDK2 to detect OP-TEE using 
SMC? This would be fully generic and presumably be the first step in 
having an EFI OP-TEE driver.


Daniel.