[PATCH v3 1/6] docs: staging/tee.rst: add a section on OP-TEE notifications

[Marc Zyngier]

On Tue, 27 Jul 2021 08:46:39 +0100,
Jens Wiklander <jens.wiklander at linaro.org> wrote:
> 
> On Fri, Jul 23, 2021 at 12:16 PM Marc Zyngier <maz at kernel.org> wrote:
> >
> > On Fri, 23 Jul 2021 10:44:17 +0100,
> > Jens Wiklander <jens.wiklander at linaro.org> wrote:
> > >
> > > Adds a section on notifications used by OP-TEE, synchronous and
> > > asynchronous.
> > >
> > > Signed-off-by: Jens Wiklander <jens.wiklander at linaro.org>
> > > ---
> > >  Documentation/staging/tee.rst | 27 +++++++++++++++++++++++++++
> > >  1 file changed, 27 insertions(+)
> > >
> > > diff --git a/Documentation/staging/tee.rst b/Documentation/staging/tee.rst
> > > index 4d4b5f889603..37bdd097336f 100644
> > > --- a/Documentation/staging/tee.rst
> > > +++ b/Documentation/staging/tee.rst
> > > @@ -184,6 +184,33 @@ order to support device enumeration. In other words, OP-TEE driver invokes this
> > >  application to retrieve a list of Trusted Applications which can be registered
> > >  as devices on the TEE bus.
> > >
> > > +OP-TEE notifications
> > > +--------------------
> > > +
> > > +There are two kinds of notifications that secure world can use to make
> > > +normal world aware of some event.
> > > +
> > > +1. Synchronous notifications delivered with ``OPTEE_RPC_CMD_NOTIFICATION``
> > > +   using the ``OPTEE_RPC_NOTIFICATION_SEND`` parameter.
> > > +2. Asynchronous notifications delivered with a combination of a non-secure
> > > +   interrupt and a fast call from the non-secure interrupt handler.
> > > +
> > > +Synchronous notifications are limited by depending on RPC for delivery,
> > > +this is only usable when secure world is entered with a yielding call via
> > > +``OPTEE_SMC_CALL_WITH_ARG``. This excludes such notifications from secure
> > > +world interrupt handlers.
> > > +
> > > +An asynchronous notification is delivered via a non-secure interrupt to an
> > > +interrupt handler registered in the OP-TEE driver. The actual notification
> > > +value are retrieved with the fast call ``OPTEE_SMC_GET_ASYNC_NOTIF_VALUE``.
> > > +
> > > +One notification value ``OPTEE_SMC_ASYNC_NOTIF_VALUE_DO_BOTTOM_HALF`` has a
> > > +special meaning. When this value is received it means that normal world is
> > > +supposed to make a yielding call ``OPTEE_MSG_CMD_DO_BOTTOM_HALF``. This
> > > +call is done from the thread assisting the interrupt handler. This is a
> > > +building block for OP-TEE OS in secure world to implement the top half and
> > > +bottom half style of device drivers.
> > > +
> >
> > What I find missing here is a description of the trigger for this
> > interrupt, and how it influences the way the kernel drivers interacts
> > with the secure side:
> >
> > - if it is edge triggered, this is 'fire and forget'. The interrupt
> >   will be consumed by the kernel handler, and whether it eventually
> >   calls into the secure side has no impact on the interrupt flow.
> >
> > - if it is level triggered, then the interrupt may be asserted until
> >   the kernel calls into the secure side, which may then drop the line
> >   level if no other requests are pending.
> >
> > These are evidently two very different flows, and you need to pick a
> > side. Note that not all interrupt controllers support both signalling
> > modes, so you are likely to leave something behind. Or you can try and
> > support both flows, but that may make the driver slightly more
> > complex.
> >
> > Either way, this needs specifying, here and in the DT binding.
> 
> In the example I'm using a level triggered interrupt which is
> triggered by writing to GICD_ISPENDR by secure world. Reading of
> GICC_IAR should clear the interrupt,the GICv2 reference manual is
> quite clear on that.

No, it merely activates it. You can't transition an interrupt from
pending to inactive (unless you clear it using GICD_ICPENDR). If you
have spotted something else in the GICv2 architecture manual, please
say so and I'll get it fixed 15 years after the facts. The fact that
GICC_IAR consumes a pending bit introduced by a write to ISPENDR is an
implementation detail, see below.

It is also a flawed approach, as this behaviour is IMPDEF on GICv3
(see 4.5 "Shared Peripheral Interrupts" in the GICv3 arch spec). Given
that GICv2 is pretty much a dead horse (TFFT!), I can't see this approach
being successful in the long run.

> So, if I understand it correctly, it will for
> this purpose work in the same way as an edge triggered interrupt. If
> this wouldn't be the case in some configuration and the interrupt must
> be cleared by some other action that would be a job for the receiver
> of OPTEE_SMC_GET_ASYNC_NOTIF_VALUE, that is, a secure world problem.
> The normal world flow should be the same.

You are assuming that the secure side will use GICD_ISPENDR, and
that's a leap of faith. An implementation should use, say, a GPIO to
drive the interrupt line and give it proper level semantics.

> Now that we describe the interrupt configuration in device tree it
> must use something that mirrors the secure world expectations. I don't
> see a point in restricting what's allowed as long it doesn't need code
> changes in the kernel too. Does this make any sense?

And that's the crucial point: what *are* the expectations of the
secure side? You seem to assume edge semantics, but that's unclear at
best.

> If I just expand a bit above explaining that the interrupt handler
> must call OPTEE_SMC_GET_ASYNC_NOTIF_VALUE as part of clearing the
> interrupt even if it might be cleared anyway in some configurations.
> Would that make it more clear, good enough even :-) ?

This is an interrupt, please document it in terms of interrupt
signalling.

- If it is level, the handler has to call into secure to observe the
  level dropping. If the driver can observe the level being low before
  calling into secure, it is perfectly allowed to consider the
  interrupt being spurious and not perform the call. If you don't have
  a device actively driving the line, this doesn't work.

- It is edge, the handler can do anything it likes, including ignoring
  the request after consuming the interrupt, or call into secure from
  a kernel thread with interrupts enabled.

At the end of the day, only you can decide which of these two flows
are appropriate. If you don't want to mandate actual HW driving the
line, edge triggered is your only option.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.