Re: [PATCH v4 5/5] xen/arm: ffa: Enable VM to VM without firmware

[Julien Grall <julien@xxxxxxx>]

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 ...
> 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)?
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?
Cheers,

--
Julien Grall