This is the third part in a series looking in detail at the updated features of the UEFI 2.4 specification. This week we look at a new restrictions placed on UEFI variables. UEFI variables services provide a system-wide repository for data, either volatile (that is, they are lost at power-off or reset) or non-volatile (that is, they are saved across a power-off or reset).

Each UEFI variable has a name composed from two parts: a VendorGuid and a VariableName. The two-part scheme allows different vendors (e.g. OEMs, silicon, software or BIOS vendors) to create their own variables without fear of conflicting with a name already chosen by someone else. The main way to avoid this is conflict is for each vendor to choose their own GUID.

The UEFI specification (chapter 3) defines some variables and most of them use the GUID EFI_GLOBAL_VARIABLE. What had happened, historically, is that some 3rd parties and BIOS vendors started to use this GUID for their own variables. This defeats one of the key design objectives of the UEFI variable names.

So, with UEFI 2.4, the specification now says that "Implementations must only permit the creation of variables with a UEFI Specification-defined VendorGuid when these variables are documented in the UEFI Specification." (section 3.2) This will cause an error to be returned.

While EFI_GLOBAL_VARIABLE is the most common VendorGuid in the UEFI Specification, it is not the only one. There is the EFI_HARDWARE_ERROR_VARIABLE GUID (for hardware error records, section 7.2.3) and EFI_IMAGE_SECURITY_DATABASE GUID (for the secure boot related variables, section 27.5.3).

By adding this restriction, the UEFI-defined variable names will not conflict with vendor-specific variable names in the future. I think that similar rules will be followed for the EFI_ prefix for symbol names. There have been a lot of implementations, include tianocore.org, that use EFI_ for their own purposes. In the PI specification, this led to problems when the names that the UEFI specification wanted had already been used by a popular implementation. This proactive step by UEFI 2.4 prevents this issue for UEFI variables.Tim LewisInsyde Software CTORepublished with Permission from the UEFI Blog
<< Back to Blogging Insyde