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Re: [PATCH v4 5/5] xen/arm: ffa: Enable VM to VM without firmware
- To: Julien Grall <julien@xxxxxxx>
- From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
- Date: Thu, 27 Mar 2025 08:37:01 +0000
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- Delivery-date: Thu, 27 Mar 2025 08:37:23 +0000
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- Thread-topic: [PATCH v4 5/5] xen/arm: ffa: Enable VM to VM without firmware
Hi Julien,
> On 27 Mar 2025, at 00:41, Julien Grall <julien@xxxxxxx> wrote:
>
> Hi Bertrand,
>
> On 24/03/2025 13:53, Bertrand Marquis wrote:
>> When VM to VM support is activated and there is no suitable FF-A support
>> in the firmware, enable FF-A support for VMs to allow using it for VM to
>> VM communications.
>
> tee/ and the callbacks associated are meant to be used for mediatiors. My
> current interpretation ist this is only meant to interpose between a guest
> and physical resources. Here you are extending the meaning to "virtual TEE".
> I am sort of ok with that but ...
I see what you mean but FF-A will not only be used to communicate with TEE
(even if the primary use case right now is this one, including have it in a VM
instead of the secure world).
>
>> If there is OP-TEE running in the secure world and using the non FF-A
>> communication system, having CONFIG_FFA_VM_TO_VM could be non functional
>> (if optee is probed first) or OP-TEE could be non functional (if FF-A is
>> probed first) so it is not recommended to activate the configuration
>> option for such systems.
>
> ... this part is concerning me. You should be able to build with
> CONFIG_FFA_VM_TO_VM and still boot when OP-TEE is present on the system. This
> is not too critical now as this is tech preview but this is definitely a
> blocker for making FFA supported. Can this be mentioned at the top of the
> ffa.c file (which already contains existing blocker)?
OP-TEE supports FF-A and in fact should be switched to using it by default as
it allows it to run in parallel of other TEEs in the secure world or have FF-A
compliant SPs running on top of OP-TEE.
More and more you will see FF-A popping up as a recommended (or required) part
of the firmware features.
So the only reason to use OP-TEE without FF-A is if you have an old OP-TEE in
which case your firmware will not support FF-A and using it between VMs is
probably not required.
>
> Also, given this would expose a fully virtual TEE, we should be able to have
> a system where you have some VMs talking FFA and some using the physical
> OPTEE (or another TEE). Whether we want to support it is a different question
> but this design would prevent it. Is this intended?
Right now i would say this is definitely not something we need as part of the
tech preview as anybody using this feature in Xen would use an OP-TEE with FF-A
support.
But from Xen point of view, we should (if we can) support running on old
systems with an existing OP-TEE but still use FF-A between VMs.
This has some consequences on how the tee mediator and FF-A support is designed
and I will definitely give it some thoughts (primary idea would be to decouple
FF-A with secure as mediator to FF-A between VMs somehow).
For the review side of things, am I right to understand from your comments that
this ok for now as tech-preview ?
Cheers
Bertrand
>
> Cheers,
>
> --
> Julien Grall
>
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