UEFI Variable Runtime Cache

The UEFI Variable Runtime Cache feature was introduced to reduce the total number of SMIs triggered and therefore total system time in SMM when SMM UEFI variables are enabled.

• Pros:
  • Improved system boot time
  • Improved system stability
  • Less SMI impact on the operating system (of particular importance in real-time operating systems)
• Cons:

  • Increase in runtime system memory

    Note: The memory increase is relatively minimal where the increase size is equal to the sum of the size of all non-volatile UEFI variable stores on the platform. Typically there is just one non-volatile variable store.

The feature is enabled and disabled by the following FeaturePCD in MdeModulePkg:

  ## Indicates if the UEFI variable runtime cache should be enabled.
  #  This setting only applies if SMM variables are enabled. When enabled, all variable
  #  data for Runtime Service GetVariable () and GetNextVariableName () calls is retrieved
  #  from a runtime data buffer referred to as the "runtime cache". An SMI is not triggered
  #  at all for these requests. Variables writes still trigger an SMI. This can greatly
  #  reduce overall system SMM usage as most boots tend to issue far more variable reads
  #  than writes.<BR><BR>
  #   TRUE  - The UEFI variable runtime cache is enabled.<BR>
  #   FALSE - The UEFI variable runtime cache is disabled.<BR>
  # @Prompt Enable the UEFI variable runtime cache.
  gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache|TRUE|BOOLEAN|0x00010039

The default PCD value is TRUE. The feature can be disabled by simply setting the PCD value to "FALSE" in a platform DSC file.