Question regarding maximum allocatable shared memory size before CA → TA command invocation

[ydonghyuk]

Hello,
I have a question regarding the maximum allocatable shared memory size when using TEEC_InvokeCommand.

In the following example:

op.paramTypes = (uint32_t)TEEC_PARAM_TYPES(
    TEEC_MEMREF_TEMP_INPUT,
    TEEC_MEMREF_TEMP_OUTPUT,
    TEEC_NONE,
    TEEC_NONE
);

op.params[0].tmpref.buffer = (void*)send_buf;
op.params[0].tmpref.size   = req_size;

op.params[1].tmpref.buffer = (void*)recv_buf;
op.params[1].tmpref.size   = rsp_size;

res = TEEC_InvokeCommand(&sess, (uint32_t)cmd, &op, &err_origin);

I am unsure how to determine appropriate values for
op.params[0].tmpref.size and op.params[1].tmpref.size.

Specifically:

How can I determine the maximum allowable buffer size for these temporary memory references?
Are these limits related to CFG_SHMEM_SIZE, or are there other factors that define the usable size?

I would greatly appreciate any guidance or best practices on calculating or choosing these buffer sizes.
Thank you in advance for your help.

[jenswikl]

The GlobalPlatform TEE Client API specification mentions TEEC_CONFIG_SHAREDMEM_MAX_SIZE:

The minimum allowed value for this maximum size is 512 kB (0x80000 bytes).

OP-TEE usually supports more, but probably not by an order of magnitude, using the default configuration. If you need more you'll need to tweak the configuration. CFG_SHMEM_SIZE is a factor if you don't use dynamic shared memory. What size are you aiming for?

[ydonghyuk]

@jenswi-linaro
Thanks for your support.

I have been using a setting of 256K so far.
My goal was to reach at least 512K, and I am inquiring to check if even larger sizes can be supported.

Upon checking, my project is currently configured as follows:

#define CFG_SHMEM_SIZE 0x00200000
#define CFG_CORE_DYN_SHM 1

To summarize, my understanding is as follows:

1. By specification, at least 512K is supported according to the GlobalPlatform TEE Client API.
2. CFG_SHMEM_SIZE must also be taken into account.
3. If dynamic shared memory is enabled, it may be possible to use even larger buffers.

Could you please advise on how to modify the default configuration in OP-TEE?