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

[Abdellatif El Khlifi]

Hi Mark,

On Thu, Dec 14, 2023 at 05:47:42PM +0100, Mark Kettenis wrote:
> > 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.

That's my preferred solution, thanks.

Cheers,
Abdellatif