Re: [PATCH v17 09/10] arm_ffa: efi: introduce FF-A MM communication

Ilias Apalodimas
2023-07-31T09:38:26Z

HI Tom,

On Fri, Jul 28, 2023 at 09:54:15AM -0400, Tom Rini wrote:
> On Fri, Jul 28, 2023 at 02:00:25PM +0300, Ilias Apalodimas wrote:
> > Hi Tom
> >
> > On Thu, 27 Jul 2023 at 19:43, Tom Rini <trini@konsulko.com> wrote:
> > >
> > > On Thu, Jul 27, 2023 at 05:07:11PM +0100, Abdellatif El Khlifi wrote:
> > >
> > > > Add MM communication support using FF-A transport
> > > >
> > > > This feature allows accessing MM partitions services through
> > > > EFI MM communication protocol. MM partitions such as StandAlonneMM
> > > > or smm-gateway secure partitions which reside in secure world.
> > > >
> > > > An MM shared buffer and a door bell event are used to exchange
> > > > the data.
> > > >
> > > > The data is used by EFI services such as GetVariable()/SetVariable()
> > > > and copied from the communication buffer to the MM shared buffer.
> > > >
> > > > The secure partition is notified about availability of data in the
> > > > MM shared buffer by an FF-A message (door bell).
> > > >
> > > > On such event, MM SP can read the data and updates the MM shared
> > > > buffer with the response data.
> > > >
> > > > The response data is copied back to the communication buffer and
> > > > consumed by the EFI subsystem.
> > > >
> > > > MM communication protocol supports FF-A 64-bit direct messaging.
> > > >
> > > > Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
> > > > Tested-by: Gowtham Suresh Kumar <gowtham.sureshkumar@arm.com>
> > > > Reviewed-by: Simon Glass <sjg@chromium.org>
> > > > Cc: Tom Rini <trini@konsulko.com>
> > > > Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> > > > Cc: Jens Wiklander <jens.wiklander@linaro.org>
> > > >
> > > > ---
> > > >
> > > > Changelog:
> > > > ===============
> > > >
> > > > v17:
> > > >
> > > > * show a debug message rather than an error when FF-A is not detected
> > > [snip]
> > > > diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
> > > > index c5835e6ef6..8fbadb9201 100644
> > > > --- a/lib/efi_loader/Kconfig
> > > > +++ b/lib/efi_loader/Kconfig
> > > > @@ -55,13 +55,53 @@ config EFI_VARIABLE_FILE_STORE
> > > > stored as file /ubootefi.var on the EFI system partition.
> > > >
> > > > config EFI_MM_COMM_TEE
> > > > - bool "UEFI variables storage service via OP-TEE"
> > > > - depends on OPTEE
> > > > + bool "UEFI variables storage service via the trusted world"
> > > > + depends on OPTEE && ARM_FFA_TRANSPORT
> > >
> > > You didn't get my changes in here however. If you can do EFI_MM_COMM_TEE
> > > without ARM_FFA_TRANSPORT (as lx2160ardb_tfa_stmm_defconfig does) then
> > > you don't make this option depend on . If FF-A is only
> > > for use here, you make FF-A depend on this, and the FF-A specific
> > > variable depend on ARM_FFA_TRANSPORT.
> >
> > Abdellatif hinted at what's going on here. When I added this Kconfig
> > option to lx2160 FF-A wasn't implemented yet.
>
> The defconfig has existed since May 2020, which is when you added
> EFI_MM_COMM_TEE itself too. So I think it's that no one did the check I
> did until now and saw this series was disabling what was on the other
> platform.
>
> > Since FF-A isn't a new
> > communication mechanism but builds upon the existing SMCs to build an
> > easier API, I asked Abdellatif to hide this complexity.
> > We had two options, either make Kconfig options for either FF-A or the
> > traditional SMCs and remove the dependencies, or piggyback on FF-As
> > discovery mechanism and make the choice at runtime. The latter has a
> > small impact on code size, but imho makes developers' life a lot
> > easier.
>
> I'm not sure how much you can do a run-time option here since you're
> setting a bunch of default values for FF-A to 0 in Kconfig. If we're
> supposed to be able to get them at run time, we shouldn't need a Kconfig
> option at all. I'm also not sure how valid a use case it is where we
> won't know at build time what the rest of the firmware stack supports
> here.
>

That's a fair point. FF-A in theory has APIs to discover memory.
Abdellatif, why do we need the Kconfigs for shared memory on FF-A?

Regards
/Ilias
> --
> Tom