We don't have agreement yet on what the behaviour should be if non-volatile variables cannot be updated or created at runtime. Add a paragraph discussing the factors involved and asking for feedback.
Cc: Dong Wei dong.wei@arm.com Cc: Alexander Graf agraf@suse.de Cc: Peter Robinson pbrobinson@redhat.com Signed-off-by: Grant Likely grant.likely@arm.com --- source/chapter2-uefi.rst | 72 ++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 64 insertions(+), 8 deletions(-)
diff --git a/source/chapter2-uefi.rst b/source/chapter2-uefi.rst index 7b6843e..10448fd 100644 --- a/source/chapter2-uefi.rst +++ b/source/chapter2-uefi.rst @@ -197,13 +197,69 @@ command: Operating Systems calls to reset the system will go via Runtime Services and not directly to PSCI.
-Set Variable ------------- +Runtime Variable Access +----------------------- + +.. todo: + + There are many platforms where it is difficult to support SetVariable() for + non-volatile variables because the firmware cannot access storage after + ExitBootServices() is called. + e.g., If firmware accesses an eMMC device directly at runtime, it will + collide with transactions initiated by the OS. + Neither U-Boot nor Tianocore have a solution for accessing shared media for + variable updates. [#OPTEESupplicant]_ + + In these platforms SetVariable() calls with the EFI_VARIABLE_NON_VOLATILE + attribute set will work in boot services, but will fail in runtime services. + The [UEFI]_ specification doesn't address what to do in this situation. + We need feedback on options before writing this section of EBBR, or making a + proposal to modify UEFI. + + We need a solution that communicates to the OS that non-volatile variable + updates are not supported at runtime, and that defines the behaviour when + SetVariable() is called with the EFI_VARIABLE_NON_VOLATILE attribute. + + Presumably, the solution will require SetVariable() to return + EFI_INVALID_PARAMETER if called with the EFI_VARIABLE_NON_VOLATILE + attribute, but beyond that there are a number of options: + + #. Clear EFI_VARIABLE_NON_VOLATILE from all variables at ExitBootServices() + + If the platform is incapable of updating non-volatile variables from Runtime + Services then it must clear the EFI_VARIABLE_NON_VOLATILE attribute from all + non-volatile variables when ExitBootServices() is called. + + An OS can discover that non-volatile variables cannot be updated at + runtime by noticing that the NON_VOLATILE attribute is not set. + + #. Clear all variables at ExitBootServices() + + If the platform is incapable of updating non-volatile variables from Runtime + Services then it will clear all variables and return EFI_INVALID_PARAMETER + on all calls to SetVariable(). + + SUSE in particular currently uses this behaviour to decide whether or not + to treat the ESP as removable media. + + #. Advertise that SetVariable() doesn't work at runtime with another variable + + Platforms can check another variable to determine if they have this quirk, + perhaps by adding a new BootOptionSupport flag. + + This is not a complete list, and other options can still be proposed. We're + looking for feedback on what would be most faithful to the UEFI spec, and + would work for the OS distributions before filling out this section of the + specification. + + Comments can be sent to the boot-architecture@lists.linaro.org mailing list.
-Non-volatile UEFI variables must persist across reset, and emulated variables -in RAM are not permitted. -The UEFI Runtime Services must be able to update the variables directly without -the aid of the Operating System. +.. [#OPTEESupplicant] It is worth noting that OP-TEE has a similar problem + regarding secure storage. + OP-TEE's chosen solution is to rely on an OS supplicant agent to perform + storage operations on behalf of OP-TEE. + The same solution may be applicable to solving the UEFI non-volatile + variable problem, but that approach is also not entirely UEFI compliant + because it requires additional OS support to work.
-.. note:: This normally requires dedicated storage for UEFI variables that is - not directly accessible from the Operating System. + https://github.com/OP-TEE/optee_os/blob/master/documentation/secure_storage....