Re: [PATCH v6 3/4] tee: optee: support tracking system threads

On Fri, 12 May 2023 at 10:27, Etienne Carriere
[off-list ref] wrote:

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On Thu, 11 May 2023 at 13:31, Sumit Garg [off-list ref] wrote:

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On Thu, 11 May 2023 at 13:49, Etienne Carriere
[off-list ref] wrote:

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On Thu, 11 May 2023 at 09:27, Sumit Garg [off-list ref] wrote:

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+bool optee_cq_inc_sys_thread_count(struct optee_call_queue *cq)
+{
+       bool rc = false;
+
+       mutex_lock(&cq->mutex);
+
+       /* Leave at least 1 normal (non-system) thread */

IMO, this might be counter productive. As most kernel drivers open a
session during driver probe which are only released in the driver
release method.

It is always the case?

This answer of mine is irrelevant. Sorry,
Please read only the below comments of mine, especially:
| Note that an OP-TEE thread is not bound to a TEE session but rather
| bound to a yielded call to OP-TEE.

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If the kernel driver is built-in then the session is
never released. Now with system threads we would reserve an OP-TEE
thread for that kernel driver as well which will never be available to
regular user-space clients.

That is not true. No driver currently requests their TEE thread to be
a system thread.
Only SCMI does because it needs to by construction.

Yes that's true but what prevents future/current kernel TEE drivers
from requesting a system thread once we have this patch-set landed.

Only clients really needing this system_thread attribute should request it.
If they really need, the OP-TEE firmware in secure world should
provision sufficient thread context.

How do we quantify it? We definitely need a policy here regarding
normal vs system threads.

One argument in favor of kernel clients requiring system threads could
be that we don't want to compete with user-space for OP-TEE threads.

Sorry I don't understand. What do you mean qualifying this?

I mean we have to fairly allocate threads among system and non-system
thread invocations.

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In an ideal situation, we would have OP-TEE provisioned with largely
sufficient thread contexts. However there are systems with constraints
memory resource that do lower at most the number of OP-TEE thread
contexts.

Yeah, I think we are on the same page here.

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So I would rather suggest we only allow a
single system thread to be reserved as a starting point which is
relevant to this critical SCMI service. We can also make this upper
bound for system threads configurable with default value as 1 if
needed.

Note that SCMI server can expose several SCMI channels (at most 1 per
SCMI protocol used) and each of them will need to request a
system_thread to TEE driver.

Etienne

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Reserving one or more system threads depends on the number of thread
context provisioned by the TEE.
Note that the implementation proposed here prevents Linux kernel from
exhausting TEE threads so user space always has at least a TEE thread
context left available.

Yeah but on the other hand user-space clients which are comparatively
larger in number than kernel clients. So they will be starved for
OP-TEE thread availability. Consider a user-space client which needs
to serve a lot of TLS connections just waiting for OP-TEE thread
availability.

Note that OP-TEE default configuration provisions (number of CPUs + 1)
thread context, so the situation is already present before these
changes on systems that embedded an OP-TEE without a properly tuned
configuration. As I said above, Linux kernel cannot be responsible for
the total number of thread contexts provisioned in OP-TEE. If the
overall system requires a lot of TEE thread contexts, one should embed
a suitable OP-TEE firmware.

Wouldn't the SCMI deadlock problem be solved with just having a lot of
OP-TEE threads? But we are discussing the system threads solution here
to make efficient use of OP-TEE threads. The total number of OP-TEE
threads is definitely in control of OP-TEE but the control of how to
schedule and efficiently use them lies with the Linux OP-TEE driver.

So, given our overall discussion in this thread, how about the upper
bound for system threads being 50% of the total number of OP-TEE
threads?

What would be a shame if the system does not use any Linux kernel
client sessions, only userland clients. This information cannot be
knwon be the linux optee driver.
Instead of leaving at least 1 TEE thread context for regular session,
what if this change enforce 2? or 3? Which count?
I think 1 is a fair choice: it allows to support OP-TEE firmwares with
a very small thread context pool (when running in small secure
memory), embedding only 2 or 3 contextes.

IMO, leaving only 1 thread for user-space will starve TLS based
applications. How about the following change on top of this patchset?

diff --git a/drivers/tee/optee/call.c b/drivers/tee/optee/call.c
index 8b8181099da7..1deb5907d075 100644
--- a/drivers/tee/optee/call.c
+++ b/drivers/tee/optee/call.c

@@ -182,8 +182,8 @@ bool optee_cq_inc_sys_thread_count(struct

optee_call_queue *cq)

        mutex_lock(&cq->mutex);

-       /* Leave at least 1 normal (non-system) thread */
-       if (cq->res_sys_thread_count + 1 < cq->total_thread_count) {
+       /* Leave at least 50% for normal (non-system) threads */
+       if (cq->res_sys_thread_count < cq->total_thread_count/2) {
                cq->free_normal_thread_count--;
                cq->res_sys_thread_count++;
                rc = true;

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Note that an OP-TEE thread is not bound to a TEE session but rather
bound to a yielded call to OP-TEE.

tee_client_open_session()
  -> optee_open_session()

tee_client_system_session()
  -> optee_system_session()
    -> optee_cq_inc_sys_thread_count()       <- At this point you
reserve a system thread corresponding to a particular kernel client
session

All tee_client_invoke_func() invocations with a system thread capable
session will use that reserved thread.

tee_client_close_session()
  -> optee_close_session()
    -> optee_close_session_helper()
      -> optee_cq_dec_sys_thread_count()    <- At this point the
reserved system thread is released

Haven't this tied the system thread to a particular TEE session? Or am
I missing something?

These changes do not define an overall single system thread.
If several sessions requests reservation of TEE system thread, has
many will be reserved.
Only the very sessions with its sys_thread attribute set will use a
reserved thread. If such a kernel client issues several concurrent
calls to OP-TEE over that session, it will indeed consume more
reserved system threads than what is actually reserved. Here I think
it is the responsibility of such client to open as many sessions as
requests. This is what scmi/optee driver does (see patch v6 4/4). An
alternative would be to have a ref count of sys_thread in session
contexts rather than a boolean value. I don't think it's worth it.

Ah, I missed that during the review. The invocations with system
threads should be limited by res_sys_thread_count in a similar manner
as we do with normal threads via free_normal_thread_count. Otherwise,
it's unfair for normal thread scheduling.

I suppose there isn't any interdependency among SCMI channels itself
such that a particular SCMI invocation can wait until the other SCMI
invocation has completed.

I think that would over complexify the logic.

We shouldn't allow system thread invocations to be greater than what
is actually reserved count for system threads. One thing I am not able
to understand here is why do you need a lot of system threads? Are
SCMI operations too expensive? I suppose those should just involve
configuring some register bits and using a single OP-TEE thread which
is invoked sequentially should be enough.