Re: [RFC PATCH 0/3] initalise ff-a after finalising pKVM

[Yeoreum Yun <yeoreum.yun@arm.com>]

> Hi Levi,
> 
> On 5/5/26 12:16, Yeoreum Yun wrote:
> >> Hi Ben,
> >>
> >>> Hi Levi,
> >>>
> >>> On 5/5/26 10:54, Yeoreum Yun wrote:
> >>>> This patch is split out from the patchset [0] --
> >>>> fix FF-A call failure with pKVM when the FF-A driver is built-in,
> >>>> specifically the IMA-related part.
> >>>>
> >>>> When pKVM is enabled, the FF-A driver must be initialised after pKVM.
> >>>> Otherwise, pKVM cannot negotiate the FF-A version or obtain the RX/TX
> >>>> buffer information, leading to failures in FF-A calls.
> >>>>
> >>>> Currently, pKVM initialisation completes at device_initcall_sync,
> >>>> while ffa_init() runs at the device_initcall level.
> >>>>
> >>>> So far, linker deployes kvm_arm_init() before ffa_init(), and SMCs can
> >>>> still be trapped even before finalise_pkvm() is invoked.
> >>>> As a result, this issue has not been observed.
> >>>>
> >>>> However, relying on above stuff is fragile.
> >>>> Therefore, when pKVM is enabled, the FF-A infrastructure should be
> >>>> initialised only after pKVM initialisation has been fully finalised.
> >>>>
> >>>> To achieve this, introduce an ffa_root_dev ("arm-ffa") and
> >>>> a corresponding driver to defer initialisation of the FF-A infrastructure
> >>>> until pKVM initialisation is complete, and to defer probing of all FF-A devices until then
> >>>> when pKVM is enabled.
> >>>>
> >>>> This patch is based on v7.1-rc2
> >>>>
> >>>> Question:
> >>>>
> >>>> FF-A initialisation can occur at late_initcall. Because it may be deferred,
> >>>> some FF-A requests cannot be serviced at that stage.
> >>>> A typical example is the EFI runtime variable service using DIRECT_MSG_REQ.
> >>>>
> >>>> Depending on the platform, the EFI runtime variable service runs with StandaloneMm
> >>>> and uses FF-A DIRECT_REQ. However, when pKVM is enabled, FF-A initialisation
> >>>> may be deferred to late_initcall. In this case, load_uefi_certs()
> >>>> can fail if it is invoked before the FF-A driver is initialised
> >>>> via deferred_probe_initcall().
> >>>>
> >>>> Moving load_uefi_certs() to late_initcall_sync, as in the third patch,
> >>>> seems not to have any problem since late_initcall and
> >>>> late_initcall_sync are both of do_basic_setup() and it's before loading
> >>>> init process. However, it is still unclear whether
> >>>> it would be better to allow DIRECT_MSG_REQ in kvm_host_ffa_handler()
> >>>
> >>> The spec doesn't allow this. Looking at DEN0077A 1.2 REL0:
> >>>
> >>> Section 13.2.2 says:
> >>>
> >>> "If they are compatible, it enables them to determine which Framework functionalities can be used. Hence, negotiation of
> >>> the version must happen before an invocation of any other FF-A ABI."
> >>>
> >>> and a bit further down
> >>>
> >>> "Once the caller invokes any FF-A ABI other than FFA_VERSION, the version negotiation phase is complete."
> >>>
> >>> I would have thought that an SP would only go into the waiting state once the version negotiation is done.
> >>
> >> I mean the negotiation between hypervisor and ff-a driver.
> >> actually the version negotiation is done with SPMC in
> >> hyp_ffa_init() but the negotiaion between hypervisor and ff-a driver
> >> just choose the lower version between version requested from ff-a driver
> >> and negotiated version with hypervisor and SPMC.
> > 
> > Sorry. re-parse the word, not choose "re-negotiate" when
> > FF-A driver request lowever version.
> > 
> >>
> >> So, the version negotiation is already done with SPMC
> >> but with FF-A driver with hypervisor is not yet.
> >> However, DIRECT_MSG_REQ has supported from v1.0 
> >> In this situation, is there any reason not to send DIRECT_REQ_MSG?
> > 
> > IOW, question is that some of ff-a request can be allowed
> > before version negotiation with FF-A driver but
> > using negotiated version via hyp_ffa_init() first or not.
> 
> I don't think so. Isn't it more a continuation of the negotiation rather than a re-negotiation?

Might be. However, in the case I mentioned, I’m asking because
it’s somewhat unusual in that the FF-A request occurs without an “FF-A driver.”

If the FF-A request goes through the FF-A driver, then as you said,
it can reasonably be considered a continuation of the negotiation.
But in this case, I was wondering whether it would be acceptable to
introduce additional exception handling for situations
where an FF-A request occurs without the FF-A driver.

From that perspective, even if the FF-A request does not go through
the FF-A driver, it would ultimately still have to wait until
the FF-A driver initialization is complete.

So my question was whether certain operations could be handled
as exceptions in such cases.

Thanks.

-- 
Sincerely,
Yeoreum Yun