Adding EFI runtime support to the Arm's FF-A bus

[Mark Kettenis]

> Date: Thu, 14 Dec 2023 15:53:46 +0000
> From: Abdellatif El Khlifi <abdellatif.elkhlifi at arm.com>

Hi Abdellatif,

> Hi guys,
> 
> I'd like to ask for advice regarding adding EFI RT support to the Arm's FF-A bus
> in U-Boot.
> 
> The objective is to enable the FF-A messaging APIs in EFI RT to be
> used for comms with the secure world. This will help getting/setting
> EFI variables through FF-A.
> 
> The existing FF-A APIs in U-Boot call the DM APIs (which are not available at RT).
> 
> Two possible solutions:
> 
> 1/ having the entire U-Boot in RT space (as Simon stated in this discussion[1])

I don't think this is a terribly good idea.  With this approach orders
of magnitude more code will be present in kernel address space one the
OS kernel is running and calling into the EFI runtime.  Including code
that may access hardware devices that are now under OS control.  It
will be nigh impossible to audit all that code and make sure that only
a safe subset of it gets called.  So...

> 
> 2/ Create an RT variant for the FF-A APIs needed.
>       These RT variant don't call the DM APIs
>       (e.g: ffa_mm_communicate_runtime, ffa_sync_send_receive_runtime, ...)
> 
> What do you recommend please ?

...this is what I would recommend.  Preferably in a way that refactors
the code such that the low-level functionality is shared between the
DM and non-DM APIs.

> 
> [1]: https://lore.kernel.org/all/CAPnjgZ21qLnSFcCJ9rPQXEfQf-NjMBF3kYUDBBOc6WUQXQtDEA@mail.gmail.com/
> 
> Cheers,
> Abdellatif
>