Re: [Xen-devel] [PATCH v6 09/10] tools/arm: tee: add "tee" option for xl.cfg

[Julien Grall <julien.grall@arm.com>]

On 6/18/19 3:30 PM, Volodymyr Babchuk wrote:
> 
> 
> Julien Grall writes:
> 
>> On 18/06/2019 12:19, Volodymyr Babchuk wrote:
>>>
>>> Hi Julien,
>>
>> Hi,
>>
>>>
>>> Julien Grall writes:
>>>>> +
>>>>> +=item B<optee>
>>>>> +
>>>>> +Allow a guest to use OP-TEE. Note that a virtualization-aware OP-TEE
>>>>> +is required for this. If this option is selected, guest will be able
>>>>
>>>> OOI, what happen if OP-TEE does not support virtualization. Will Xen
>>>> forbid to use it?
>>> Yes, Xen will get an error from OP-TEE during domain construction. This
>>> will lead to domain creation failure.
>>
>> This is a bit odd. It means we have no way to know in advance whether
>> OP-TEE will be able to create a client.
> Yes. There can be at least two reasons for this:
> 1. OP-TEE is built without virtualization support at all
> 2. OP-TEE have no resources for a new guest
> 
>> In other word, when the
>> mediator is built in Xen, all existing setup with OP-TEE (and
>> no-virtualization) will fail.
> Right. If user provides DTB with 'optee' node, but OP-TEE is build without
> virtualization support, Dom0 will not be created. This can be fixed by
> adding new capability flag into OP-TEE, that tells Xen about
> virtualization support. For some reason I missed this when I implemented
> VM support in OP-TEE :(
> 
>> My expectation is Xen should be able to know whether the mediator can be used.
> I need to implement additional capability flag in the OP-TEE. This is
> not so hard, but it will be available only in the next release. For now,
> we can document this limitation somewhere.

Is OP-TEE already released with virtualization? If not, when will it be?

> 
>>>
>>>>> +to access to the real OP-TEE OS running on the host. Guest creation
>>>>
>>>> s/real// it is redundant with the rest of the sentence. However, it
>>>> does not really answer to the question regarding isolation.
>>> Your assumption is correct - OP-TEE provides isolation on its side.
>>>
>>>>
>>>>> +will fail if OP-TEE have no resources for a new guest. Number of supported
>>>>> +guests depends on OP-TEE configuration.
>>>>
>>>> How about the following description (correct me if my understanding is
>>>> wrong):
>>>>
>>>> "Allow a guest to access the host OP-TEE OS. Xen will mediate the
>>>> access to OP-TEE and the resource isolation will be provided directly
>>>> by OP-TEE. OP-TEE itself may limit the number of guests that can
>>>> concurrently use it. This requires a virtualization-aware OP-TEE for
>>>> this to work.
>>>>
>>>> This feature is a B<technology preview>."
>>> That's much better than my version. Thank you.
>>>
>>>> How can a user know whether OP-TEE supports virtualization? Is it
>>>> configurable at build?
>>> Yes, there is a special configuration option CFG_VIRTUALIZATION. This is
>>> covered in OP-TEE documentation at [1]
>>>
>>> [1] https://optee.readthedocs.io/architecture/virtualization.html
>>
>> Do we expect the link to be stable? If so, then I think a link in the
>> documentation would be useful.
> This is the official OP-TEE documentation. So, yes, it should be stable.
> I can put this link into the code somewhere.

I would add the link in the xl documentation and also in the commit 
message of patch #2. I can do the later on commit.

Cheers,

-- 
Julien Grall

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