Re: [PATCH v2 0/7] Asynchronous notifications from secure world

[Sudeep Holla <sudeep.holla@arm.com>]

On Fri, Jul 09, 2021 at 08:05:57AM +0000, Etienne CARRIERE wrote:
> Hello Sudeep and all,
> 
> On Wed, 7 Jul 2021 at 19:52, Sudeep Holla <sudeep.holla@arm.com> wrote:
> >
> > Hi Sumit,
> >
> > I was holding off you reply as I didn't have all the background on this.
> > Achin did mention that this is preparatory work for FFA notifications.
> > I did mention to him that this is more than that, it is custom extension
> > to address what FF-A notification is trying to in standard way.
> >
> > I share same opinion as Marc Z.
> >
> > On Wed, Jul 07, 2021 at 11:22:23AM +0530, Sumit Garg wrote:
> > > On Tue, 6 Jul 2021 at 18:16, Marc Zyngier <maz@kernel.org> wrote:
> >
> > [...]
> >
> > > >
> > > > I don't care about OP-TEE. If you are proposing a contract between S
> > > > and NS, it has to be TEE and OS independent. That's how the
> > > > architecture works.
> > > >
> > >
> > > Agree, here we are not proposing a common contract among the S and NS
> > > world that every TEE (based on Arm TrustZone) will use to communicate
> > > with REE (Linux in our case) but rather an OP-TEE specific
> > > notifications feature that is built on top of OP-TEE specific ABIs.
> > >
> > > And I can see your arguments coming from an FFA perspective but there
> > > are platforms like the ones based on Armv7 which don't support FFA
> > > ABI. Maybe Jens can elaborate how this feature will fit in when FFA
> > > comes into picture?
> > >
> >
> > I can understand that but won't those platforms add the support both in
> > the kernel(current series) and secure world to address notifications.
> > While you could argue that it is small extension to what is already present
> > but I prefer they support FF-A is they need such a support instead of adding
> > custom mechanisms. It is hard to maintain and each vendor will deviate
> > from this custom mechanism and soon we will have bunch of them to handle.
>
> There exist armv7-a platforms that expect OP-TEE notification support and
> will not move the FF-A, like the stm32mp15. This platform won't move to FF-A
> mainly due to the memory cost of the added SPM layer and the device physical
> constraints.

Fair enough on the use-case and the analysis for not being able to use FF-A.
As you may already know it doesn't simply this problem. This has been
discussed for years and FF-A was assumed to be the solution when FF-A
spec work started.

> We have a usecase for OP-TEE notification. We're working on the integration
> of an SCMI server in OP-TEE. SCMI notification is a feature needed is this
> scope and it requires OP-TEE async notification means as those proposed
> here.
>

I am aware of this use-case, I understand. But I can only share rants
which I know doesn't help much.

> This OP-TEE async notif also brings a lot of value in OP-TEE as it allows a
> OP-TEE secure thread (i.e. executing a trusted application service) to
> gently wait on a secure interrupt (as a slow bus transaction completion or
> many other usecase) with the CPU relaxed. This support is provided by the
> proposed series. I believe existing device should be able to leverage this
> OP-TEE feature without needing their OP-TEE to move to the new FF-A
> interface.
>

While I agree these are nice to have in OPTEE, the timing is just odd.

We are trying hard to push FF-A as standard solution to address all such
issues that couldn't be solved with OPTEE + DT, now we are back to address
the same in parallel to FF-A.

--
Regards,
Sudeep