[TF-M] augmenting the SVC_Handler_IPC() to support custom services

I need to add custom SPM APIs to augment our SP services. Consequently, I need to extend the set of SVCs supported in SVC_Handler_IPC().

I propose to modify the SVC_Handler_IPC() function's 'default' handler to invoke a locally defined weak function such as below:

default: return (custom_ipc_svc_handlers(svc_num, ctx, lr));

__attribute__((weak)) int32_t custom_ipc_svc_handlers(tfm_svc_number_t svc_num, uint32_t *ctx, uint32_t lr) { LOG_MSG("Unknown SVC number requested!"); return PSA_ERROR_GENERIC_ERROR; }

This will allow a 'strong'ly defined custom_ipc_svc_handlers() function to be invoked if provided.

Is this OK?

Another approach is for me to hijack the root SVC handler in the secure vector table, but this seems too heavy handed to me.

Alan