On platforms using the Qualcomm UEFI Secure Application (uefisecapp),
EFI variables cannot be accessed via the standard interface in EFI
runtime mode. The respective functions return EFI_UNSUPPORTED. On these
platforms, we instead need to talk to uefisecapp. This commit provides
support for this and registers the respective efivars operations to
access EFI variables from the kernel.

Communication with uefisecapp follows the Qualcomm QSEECOM / Secure OS
conventions via the respective SCM call interface. This is also the
reason why variable access works normally while boot services are
active. During this time, said SCM interface is managed by the boot
services. When calling ExitBootServices(), the ownership is transferred
to the kernel. Therefore, UEFI must not use that interface itself (as
multiple parties accessing this interface at the same time may lead to
complications) and cannot access variables for us.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20230827211408.689076-4-luzmaximilian@gmail.comSigned-off-by: Bjorn Andersson <andersson@kernel.org>