On Tue, 6 Jul 2021 at 18:16, Marc Zyngier maz@kernel.org wrote:
Sumit,
On Tue, 06 Jul 2021 12:39:13 +0100, Sumit Garg sumit.garg@linaro.org wrote:
Hi Marc,
On Tue, 6 Jul 2021 at 16:06, Marc Zyngier maz@kernel.org wrote:
On Tue, 06 Jul 2021 08:25:26 +0100, Sumit Garg sumit.garg@linaro.org wrote:
I could recognise it's requirement from the time while I was playing with secure timer interrupt support for OP-TEE RNG driver on Developerbox. In that case I had to strip down the secure interrupt handler to a minimum that would just collect entropy and dump into the secure buffer. But with asynchronous notifications support, I could add more functionality like entropy health tests in the bottom half instead of doing those health tests while retrieving entropy from the secure world.
Given that, have you explored the possibility to leverage SGI rather than a platform specific SPI for notifying the normal world? If it's possible to leverage Architecture specific SGI for this purpose then I
What does "Architecture specific SGI" mean?
Here I meant that SGI is specific to Arm architecture and doesn't require to be specific to per platform like an SPI.
SGIs are, by definition *software* specific (the clue is in the name), and the architecture spec has *zero* say into what they are used for. It says even less when it comes to specifying cross-world signalling.
Agree.
think this feature will come automatically enabled for every platform without the need to reserve a platform specific SPI.
That old chestnut again...
Okay, can you provide reference to earlier threads?
They show up every other year. Lore is your friend.
Okay.
- How do you discover that the secure side has graced you with a Group-1 SGI (no, you can't use one of the first 8)? for both DT and ACPI?
I think the secure world can be probed
How? With what guarantees?
It can simply be a fast SMC call to OP-TEE to retrieve the SGI to be used for notification using similar SMC as OPTEE_SMC_FUNCID_GET_ASYNC_NOTIF_VALUE that Jens has used in this patch-set.
I am not sure how that would fail as we do maintain backwards compatibility with prior OP-TEE versions.
for that during the OP-TEE driver probe.
Oh, so it is only for the benefit of a single driver?
Yeah.
And I agree with you that the first 7 SGIs are already pre-occupied and I guess you remember mine patch-set that tried to leverage 8th SGI as pseudo NMI for kernel debug purposes.
I do remember, and I'm definitely not keen on spending this last SGI on this feature.
Agree and that's why we allowed that last SGI for debug purposes if it is not used anywhere else. Let's keep this discussion to the corresponding patch-set only as otherwise we would unnecessarily derail discussion for this OP-TEE specific feature.
So yes for this use-case, the secure world can reserve one of the latter 8 SGIs (8 to 15) for cross world notification and I guess your earlier work to make SGIs to be requested as normal IRQs should make it easier to implement this as well.
- How do you find which CPUs are targeted by this SGI? All? One? A subset? What is the expected behaviour with CPU hotplug? How can the NS side (Linux) can inform the secure side about the CPUs it wants to use?
For the current OP-TEE use-case, I think targeting all CPUs would be efficient.
Efficient? How? Broadcast? One of N? Random?
By efficient here I meant that we would enable that SGI for every CPU rather than a subset so that any CPU which receives a secure interrupt (PPI or SPI) would be able to raise this SGI to itself in order to notify Linux to create a thread for OP-TEE.
So wouldn't it be possible for the CPU which receives the secure interrupt to raise that SGI to self that would in turn notify the normal world (Linux) to create a thread for OP-TEE to do bottom half processing?
You are assuming that this is the way the NS side wants to work, and I question this assumption.
Actually this is the way that Jens has implemented notifications among Linux and OP-TEE using a SPI in this patch-set. The only difference with SGI is that it's a per CPU interrupt.
- Is there any case where you would instead need a level interrupt (which a SGI cannot provide)?
I think SGI should be sufficient to suffice OP-TEE notifications use-case.
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?
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:
1. how messages are conveyed between the two worlds -> OP-TEE specific ABI (yielding SMC calls). 2. how signalling is done -> OP-TEE specific ABI (fast SMC calls). 3. how CPU PM is handled -> OP-TEE is notified on PSCI CPU ON, OFF and SUSPEND calls. 4. how targeting is negotiated -> SGI would be targeted to the same CPU which receives the secure interrupt (PPI or SPI).
If you want a custom OP-TEE hack, you don't need my blessing for 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.
-Sumit
M.-- Without deviation from the norm, progress is not possible.