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

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

Hi Julien,

Julien Grall writes:

> Hi Volodymyr,
>
> On 6/11/19 7:46 PM, Volodymyr Babchuk wrote:
>> This enumeration controls TEE type for a domain. Currently there is
>> two possible options: either 'none' or 'optee'.
>>
>> 'none' is the default value and it basically disables TEE support at
>> all.
>>
>> 'optee' enables access to the OP-TEE running on a host machine. This
>> requires special OP-TEE build with virtualization support enabled.
>>
>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
>>
>> ---
>>   All the patches to optee.c should be merged together. They were
>>   split to ease up review. But they depend heavily on each other.
>>
>>   Changes from v5:
>>    - Replaced "native" with "optee" in the commit description.
>>    - Updated and extended documentation based on Julien Grall's
>>      and Ian Jackson's suggestions.
>>
>>   Changes from v4:
>>    - "native" option was replaced with "optee"
>>    - "tee" property was moved from arch-specific section to the
>>       global one. Documentation moved inside "Devices" section.
>>
>>   Changes from v3:
>>    - tee_enabled renamed to tee_type. Currently two types are supported
>>      as described in the commit message
>>    - Add LIBXL_HAVE_BUILDINFO_ARCH_ARM_TEE definition
>>
>>   Changes from v2:
>>    - Use arch.tee_enabled instead of separate domctl
>> ---
>>   docs/man/xl.cfg.5.pod.in    | 21 +++++++++++++++++++++
>>   tools/libxl/libxl.h         |  5 +++++
>>   tools/libxl/libxl_arm.c     | 13 +++++++++++++
>>   tools/libxl/libxl_types.idl |  6 ++++++
>>   tools/xl/xl_parse.c         |  9 +++++++++
>>   5 files changed, 54 insertions(+)
>>
>> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
>> index c99d40307e..e65ab6111f 100644
>> --- a/docs/man/xl.cfg.5.pod.in
>> +++ b/docs/man/xl.cfg.5.pod.in
>> @@ -1544,6 +1544,27 @@ Set maximum height for pointer device.
>>     =back
>>   +=item B<tee="STRING">
>> +
>> +B<Arm only.> Set TEE type for the guest. TEE is a Trusted Execution
>> +Environment -- separate secure OS found on some platforms. B<STRING> can be one of the:
>> +
>> +=over 4
>> +
>> +=item B<none>
>> +
>> +Disable TEE support at all. This is the default value.
> How about "Don't allow the guest to use TEE if present on the
> platform. This is the default value.".
I'm perfectly fine with this.

>> +
>> +=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.

>> +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

-- 
Best regards,Volodymyr Babchuk
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