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

Tom Rini
2023-08-01T16:19:26Z

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On Tue, Aug 01, 2023 at 05:10:08PM +0100, Abdellatif El Khlifi wrote:
> Hi guys,
>
> On Tue, Aug 01, 2023 at 11:00:57AM -0400, Tom Rini wrote:
> > > > > > > > > ...
> > > > > > > > > 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?
> > > >
> > > > The statically carved out MM shared buffer address, size and offset cannot be discovered by FF-A ABIs.
> > > > The MM communication driver in U-Boot could allocate the buffer and share it with the MM SP but
> > > > we do not implement that support currently in either U-Boot or UEFI.
> > >
> > > Ok, that's a bit unfortunate, but Tom is right. Having the FF-A
> > > addresses show up is as confusing as having Kconfig options for
> > > discrete options. The whole point of my suggestion was to make users'
> > > lives easier. Apologies for the confusion but can you bring back the
> > > ifdefs? Looking at the patch this should be minimal just use
> > > ifdef ARM_FFA_TRANSPORT and ifndef ARM_FFA_TRANSPORT.
> > >
> > > Tom you prefer that as well?
> >
> > Pending an answer to Jens' feedback, yes, going back to #ifdef's is
> > fine, especially since default values of 0 are nonsense in this case
> > (and as Heinrich's patch re SYS_MALLOC_LEN shows, dangerous since 0 !=
> > 0x0 once we do string comparisons).
> >
>
> I'd like to give some context why it's important for Corstone-1000 platform
> that the DT passed to the kernel matches the official kernel DT.

Note that we've set aside the "should this be in DT or not" question.

> There is a SystemReady IR 2.0 test checking the DT. It compares the DT
> passed by U-Boot with a reference DT (the kernel DT) . The test fails if there
> is a mismatch. So, if we add a DT node in U-Boot and the node is not upstreamed
> to the kernel DT, the DT test will fail.

This is overall good and progress.

> To be approved by the kernel DT maintainers, the node should have a use case
> in the kernel which is not the case.

This is, I believe / hope wrong. It needs to be in the dt-schema
repository, not strictly "the kernel". For example, bootph-all (etc)
are in dt-schema and so can be in the upstream kernel but are not used
in the kernel itself.

> There is a solution for this which is deleting the node we don't want to pass to
> the kernel using delete-node in the U-Boot DT.

Something like this will likely be needed, in the end, at least for some
cases. But the goal is that everything gets in to dt-schema.

[snip]
> With this we can get rid of the configs and the #defines: FFA_SHARED_MM_BUF_ADDR,
> FFA_SHARED_MM_BUF_OFFSET and FFA_SHARED_MM_BUF_SIZE.
>
> Also, we will avoid setting 0 as default values for the address, size and offset.

We just need to not have default values offered. The symbols just need
to depend on FFA so that they aren't asked when not used.

> 2/ the FF-A specific code in efi_variable_tee.c will try to find the mm-comms-buf
> reserved memory node. When found, it reads the buffer address, size and offset.
>
> 3/ adding #ifdef CONFIG_ARM_FFA_TRANSPORT in lib/efi_loader/efi_variable_tee.c
> for the FF-A specific code.
>
> 4/ make EFI_MM_COMM_TEE depends on OPTEE only
>
> What do you think guys ?

Yes, we need to do 3 and 4.

--
Tom

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