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Re: [Xen-devel] [PATCH v4 09/10] tools/arm: tee: add "tee" option for xl.cfg





On 20/03/2019 15:27, Volodymyr Babchuk wrote:

Hello Julien,

Julien Grall writes:

On 07/03/2019 21:04, Volodymyr Babchuk wrote:
From: Volodymyr Babchuk <vlad.babchuk@xxxxxxxxx>

This enumeration controls TEE type for a domain. Currently there is
two possible options: either 'none' or 'native'.

'none' is the default value and it basically disables TEE support at
all.

'native' enables access to a "real" TEE installed on a platform.

I am aware I made that suggestion. But I think the naming is not ideal
between the user and the toolstack. The question is how this is going
to fit with the S-EL2 feature where multiple TEE can run together?
You see, support for S-EL2 or support for multiple TEEs is a much broader
topic. For me, naming for configuration option is the least important
thing in this case :-)

Naming exposed to users are hard to remove. If the naming is too ambiguous, then we will have to introduce a new option later on. This is not very ideal, so it would be better if we can find something different.


Right there is no clear vision how it will ever work. I scanned through
SPCI spec and I already have a couple of questions. I need to study it
harder to make serious statements, but I already see that current
mediator framework will hardly fit into SPCI stuff. Frankly, I have
concerns that OP-TEE (or any other existing TEE) will be compatible
with SPCI-enabled systems without major rework. So, we will need to do
big overhaul anyways, when there will appear first SPCI-compatible TEEs and
secure hypervisors.

AFAIK, there is no ARMv8.4 platforms, no S-EL2 hypervisors and so on. It
will take at least couple of years before S-EL2 will hit the market. So
in my opinion it is too early to make any assumptions on how to support
all this in Xen. Lets stick to the current matters.

I am fully aware that more work will need to be done with S-EL2. I am not expecting you (or anyone else here) to come with the implementation now. My point is the naming should be chosen so it prevents ambiguity with whatever we know will come up in the future.


I'm not insisting on "native". But I can't invent something better right
now. Probably, SPCI-enabled TEE also will be considered "native" as
opposed to, say, "emulated". >
As I said, I can't come with anything better than "native". But I'm open
to any suggestions.

Well one solution is to ditch "native" and name it "optee". By giving the name you avoid ambiguity. If we ever have multiple op-tee instance running, then it could easily be extend with a comma. So you allow backward compatibility.

AFAICT, TEE also exists on other architecture. So I am wondering
whether this field should be moved out of arch_arm?
In v3 thread you asked if I see any use for x86. Because in that version
this option was in common section. Honestly, I don't see any use there,
because I have no idea how this can be implemented on x86.

On the v3, I pointed out that the documentation was in Arm section but the code was in common. When I asked a question, it does not mean I am not happy with it. It only means I am trying to understand your choice so I can make my mind on it. Sadly, you left it unanswered until now.

In this context, I am not asking you how this is going to be implemented but whether this option could potentially be used in the future for other architecture (e.g x86, RISC-V...). For instance, the option "device_tree" is part for common options despite it is only implemented on Arm.

Cheers,

--
Julien Grall

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