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

From: Julien Grall <julien.grall@arm.com>
To: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Cc: "tee-dev@lists.linaro.org" <tee-dev@lists.linaro.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	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 13:49:42 +0100	[thread overview]
Message-ID: <ead7d399-92a2-9d7f-91e4-2d6cf4c9961b@arm.com> (raw)
In-Reply-To: <87v9x36sz1.fsf@epam.com>

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. In other word, when the mediator is built in 
Xen, all existing setup with OP-TEE (and no-virtualization) will fail.

My expectation is Xen should be able to know whether the mediator can be used.

> 
>>> +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.

Cheers,

-- 
Julien Grall

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