Hi,
I would like to restart a discussion that we already had a few years ago on a thread called "SMC to intentionally trigger a panic in TF-A" (https://lists.trustedfirmware.org/archives/search?mlist=tf-a%40lists.trusted...) but that petered out without any real resolution (and resulted in me ultimately not implementing the feature I was hoping to add).
Basically, we are repeatedly stumbling over the problem that we have a use case for some platform-independent SMC API that we want to implement in TF-A, but don't have an appropriate SMC FID range to put it. My request from a few years ago was about implementing a call to intentionally trigger a panic in TF-A for test-automation purposes. Today we came up with a use case where a Trusted OS wants to query BL31 about the location of a shared log buffer: https://review.trustedfirmware.org/20478 .
Currently, the available SMC ranges are Arm, CPU, SiP, OEM, Standard, Hypervisor, TA and TOS. The SiP, OEM and TOS ranges are all specific to a single silicon vendor, OEM or trusted OS implementation, so they are not good targets to implement APIs that would make sense to be shared among multiple of these. In theory, the Standard range would probably be the right target to implement calls that are independently useful for multiple platforms / OSes... but as far as I understand, adding a new call to that range requires petitioning Arm to update the SMC calling convention itself, which is a ridiculously high bar to implement a small utility API. In practice, the only choice we have for implementing these kinds of calls is to let every OEM, SiP or TOS assign its own (different) FID for it and then write separate SMC handlers for each in TF-A that all end up calling the same underlying function... which creates a lot of unnecessary code duplication and identifier soup (especially in the case of SMCs for the non-secure OS which would then be implemented by a platform-independent Linux driver that needs a big mapping table to decide which FID to use on which platform for the same API).
I think it would be very useful if there was another range of easily allocatable FIDs that developers could just add to with a simple TF-A CL without having to go through a huge specification update process. There are still 41 OENs unused in the Arm SMCCC, and I don't think any new ones were added in the 10 years that the specification existed... so we are really not going to run out of them any time soon. If we could get even one of those OENs for this purpose, we would have 64K FIDs to use up for our small, simple platform-independent API needs, which should last us a long while. We could maybe call it the "Secure Monitor range" and say the FIDs are specific to a certain implementation of Secure Monitor (e.g. TF-A). Then there could just be a header file in the TF-A sources that serves as the authoritative FID assignment table for TF-A, and anyone with a sufficiently useful idea (subject to TF-A maintainer review) for a platform-independent API like this could add it there by just uploading a patch.
I recently argued for a similar "simple tag allocation" concept on https://github.com/FirmwareHandoff/firmware_handoff and it found support there, so I hope I'll be able to convince you that it would be useful for SMC FIDs as well?