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

From: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
To: Julien Grall <julien.grall@arm.com>
Cc: "tee-dev@lists.linaro.org" <tee-dev@lists.linaro.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>, Wei Liu <wl@xen.org>
Subject: Re: [Xen-devel] [PATCH v6 09/10] tools/arm: tee: add "tee" option for xl.cfg
Date: Tue, 18 Jun 2019 14:30:45 +0000	[thread overview]
Message-ID: <87tvcn6k4r.fsf@epam.com> (raw)
In-Reply-To: <ead7d399-92a2-9d7f-91e4-2d6cf4c9961b@arm.com>

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.

>>
>>>> +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.

-- 
Best regards, Volodymyr Babchuk
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