Hi Julius,

I see your request as two-fold
1. Having a range for TF-A specific SMC's :  Currently we are having internal discussion whether we need a range for this and where exactly it should be (probably a sub-range in std services). Personally i think it's a genuine request, in fact there are few existing functionalities which qualify to be part of this range (PMF/DebugFS) but they are kept as part of Arm SiP.
2. SMC to get CBMEM console address: As you mentioned that it is required only once and not-necessarily a runtime use case. Is it possible to implement this using Firmware Handoff mechanism? Considering the specification is almost at final stage and TF-A implementation is likely to happen in not-so-distant future.

Thanks
Manish
From: Julius Werner via TF-A <tf-a@lists.trustedfirmware.org>
Sent: 11 April 2023 23:02
To: tf-a <tf-a@lists.trustedfirmware.org>
Cc: Dan Handley <Dan.Handley@arm.com>; Soby Mathew <Soby.Mathew@arm.com>; Joanna Farley <Joanna.Farley@arm.com>; Sandrine Bailleux <Sandrine.Bailleux@arm.com>; Jeffrey Kardatzke <jkardatzke@google.com>; Sandeep Tripathy <sandeep.tripathy@broadcom.com>
Subject: [TF-A] Need a mechanism to easily define platform-independent SMC FIDs
 
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.trustedfirmware.org&q=%22SMC+to+intentionally+trigger+a+panic+in+TF-A%22)
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?
--
TF-A mailing list -- tf-a@lists.trustedfirmware.org
To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.org