Hi Sudeep,

On Wed, 7 Jul 2021 at 23:22, Sudeep Holla [off-list ref] 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:

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On Tue, 6 Jul 2021 at 18:16, Marc Zyngier [off-list ref] wrote:

[...]

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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.

Agree.

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.

I haven't had a deep dive into FF-A spec, maybe you can clarify on the
following queries regarding Armv7 compatibility:
- As you may be aware, secure monitor implementation on Armv7 is
tightly coupled to trusted OS (part of the same code base), so would
you like each trusted OS vendor to implement a common FF-A interface?
- IIRC, FF-A spec has the notion of multiple secure partitions, are
those supported on Armv7? If yes then how?

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In general, cross world SGIs are a really bad idea. Yes, some people
like them. I still think they are misguided, and I don't intend to
provide a generic request interface for this.

Okay, as I mentioned above having it specific to OP-TEE driver
requesting secure world donated SGI would work for you?

No. I want a proper architecture between secure and non-secure that
explain how messages are conveyed between the two world, how
signalling is done, how CPU PM is handled, how targeting is
negotiated. And at the end of the day, this is starting to look a lot
like FFA.

AFAIK when FFA comes in picture than OP-TEE will use the standard
interface provided by FFA ABIs but if FFA isn't supported by a
particular platform (eg. based on Armv7) then we need to rely on TEE
specific ABI like what OP-TEE currently provides:

Who are asking for this ? Can we ask them to migrate to FF-A if this
(new) notification support is needed on their platforms ? It is help to
know the requesters so that they can be included in FF-A spec discussions.

I would let Jens answer that.

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that. You'll even get to keep the pieces once it breaks. But if you
are going to invent a new universal way of signalling things across
world, you'd better start specifying things the right way, taking into
considerations systems where the interrupt controller doesn't allow
cross-world signalling.

As I mentioned above, this patch-set adds an OP-TEE specific
notifications feature. AFAIK, the interrupt controllers supported by
OP-TEE (GICv2, GICv3 etc.) don't restrict cross-world signaling.

So given the explanation above, if you still think requesting an SGI
as an IRQ by drivers isn't allowed then I am fine with the approach
that Jens has already implemented in this patch-set to use platform
specific SPI.

And I assume these platforms in question have SPI to spare and way to
trigger it from secure world ?

Yeah, that is the requirement on the platform if we rely on SPI (Qemu
test example [1]) which wouldn't be the case if we use secure world
donated SGI.

BTW, is this notification mechanism discussed in the case of FF-A? If
yes, can you throw some light on that?

[1] https://github.com/jenswi-linaro/optee_os/commit/9007f8184deb9b7995da8d590779cb3ba2783394

-Sumit

--
Regards,
Sudeep

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