Re: [PATCH v5 05/10] xen/arm: optee: add std call handling

[Julien Grall <julien.grall@arm.com>]

Hi Volodymyr,

On 21/05/2019 22:26, Volodymyr Babchuk wrote:
> The main way to communicate with OP-TEE is to issue standard SMCCC
> call. "Standard" is a SMCCC term and it means that call can be
> interrupted and OP-TEE can return control to NW before completing
> the call.
> 
> In contrast with fast calls, where arguments and return values
> are passed in registers, standard calls use shared memory. Register
> pair a1,a2 holds 64-bit PA of command buffer, where all arguments
> are stored and which is used to return data. OP-TEE internally
> copies contents of this buffer into own secure memory before accessing
> and validating any data in command buffer. This is done to make sure
> that NW will not change contents of the validated parameters.
> 
> Mediator needs to do the same for number of reasons:
> 
> 1. To make sure that guest will not change data after validation.
> 2. To translate IPAs to PAs in the command buffer (this is not done
>     in this patch).
> 3. To hide translated address from guest, so it will not be able
>     to do IPA->PA translation by misusing mediator.
> 
> During standard call OP-TEE can issue multiple "RPC returns", asking
> NW to do some work for OP-TEE. NW then issues special call
> OPTEE_SMC_CALL_RETURN_FROM_RPC to resume handling of the original call.
> Thus, mediator needs to maintain context for original standard call
> during multiple SMCCC calls.
> 
> Standard call is considered complete, when returned value is
> not a RPC request.
> 
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>

Acked-by: Julien Grall <julien.grall@arm.com>

Cheers,

-- 
Julien Grall

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