Hello Ilias,
thanks a lot for the good online meeting this morning together with your colleague Suggan where we discussed the requirements for the implementation of non-volatile variables in U-Boot.
Currently UEFI variables are stored in U-Boot variables. Saving the U-Boot variables will persist all UEFI variables in the environment both volatile and non-volatile. This does not conform the the UEFI standard.
To provide a secure storage Linaro is considering to implement an OP-TEE module for variable storage and as alternative to this OP-TEE module a standalone MM service which will be a BL32 ATF module.
So in future we will have possibly three alternative drivers for UEFI variables:
- U-Boot only implementation (what is now in lib/efi_loader/efi_variable.c) - OP-TEE module - standalone MM service
And maybe a fourth dummy one implementing no variable service at all.
To make the OP-TEE module and the standalone MM service usable in EDK2 too they will provide implementations for GetVariable(), GetNextVariable(), QueryVariable(), and SetVariable() both for volatile and non-volatile variables.
The U-Boot only implementation currently provides these variables only at boottime. Takahiro sent patches with a runtime cache enabling GetVariable(), GetNextVariable() and possibly in future QueryVariable() but not SetVariable().
Currently when reaching SetVirtualAddressMap() we patch the runtime table to replace the variable services by dummy functions returning EFI_UNSUPPORTED (or at least this should be the return value). Alex delivered a patch that will move this patching to ExitBootServices() to comply with the requirements of operating systems which do not call SetVirtualAddressMap().
As in future we will have three different drivers for variables and these will differ in which runtime services they support it makes sense to move the obligation of patching the runtime services table to the variable driver itself. We already have implemented the event group EFI_EVENT_GROUP_EXIT_BOOT_SERVICES. So if the driver creates an event for this group it can do the patching in the notification function of the event.
I will update the current variables implementation to take care of the runtime services table patching for variable services using EFI_EVENT_GROUP_EXIT_BOOT_SERVICES and add a null driver. This will provide the templates for the U-Boot side implementation of the OP-TEE and standalone MM service solution.
Once we have that API right we can start moving in Takahiros patches for separating UEFI and U-Boot variables and creating a runtime cache.
Best regards
Heinrich