mirror of https://github.com/acidanthera/audk.git
aab3b9b9a1
REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2220 This change reduces SMIs for GetVariable () by maintaining a UEFI variable cache in Runtime DXE in addition to the pre- existing cache in SMRAM. When the Runtime Service GetVariable() is invoked, a Runtime DXE cache is used instead of triggering an SMI to VariableSmm. This can improve overall system performance by servicing variable read requests without rendezvousing all cores into SMM. The runtime cache can be disabled with by setting the FeaturePCD gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache to FALSE. If the PCD is set to FALSE, the runtime cache will not be used and an SMI will be triggered for Runtime Service GetVariable () and GetNextVariableName () invocations. The following are important points regarding the behavior of the variable drivers when the variable runtime cache is enabled. 1. All of the non-volatile storage contents are loaded into the cache upon driver load. This one time load operation from storage is preferred as opposed to building the cache on demand. An on- demand cache would require a fallback SMI to load data into the cache as variables are requested. 2. SetVariable () requests will continue to always trigger an SMI. This occurs regardless of whether the variable is volatile or non-volatile. 3. Both volatile and non-volatile variables are cached in a runtime buffer. As is the case in the current EDK II variable driver, they continue to be cached in separate buffers. 4. The cache in Runtime DXE and SMM are intended to be exact copies of one another. All SMM variable accesses only return data from the SMM cache. The runtime caches are only updated after the variable I/O operation is successful in SMM. The runtime caches are only updated from SMM. 5. Synchronization mechanisms are in place to ensure the runtime cache content integrity with the SMM cache. These may result in updates to runtime cache that are the same in content but different in offset and size from updates to the SMM cache. When using SMM variables with runtime cache enabled, two caches will now be present. 1. "Runtime Cache" - Maintained in VariableSmmRuntimeDxe. Used to service Runtime Services GetVariable () and GetNextVariableName () callers. 2. "SMM Cache" - Maintained in VariableSmm to service SMM GetVariable () and GetNextVariableName () callers. a. This cache is retained so SMM modules do not operate on data outside SMRAM. Because a race condition can occur if an SMI occurs during the execution of runtime code reading from the runtime cache, a runtime cache read lock is introduced that explicitly moves pending updates from SMM to the runtime cache if an SMM update occurs while the runtime cache is locked. Note that it is not expected a Runtime services call will interrupt SMM processing since all CPU cores rendezvous in SMM. It is possible to view UEFI variable read and write statistics by setting the gEfiMdeModulePkgTokenSpaceGuid.PcdVariableCollectStatistics FeaturePcd to TRUE and using the VariableInfo UEFI application in MdeModulePkg to dump variable statistics to the console. By doing so, a user can view the number of GetVariable () hits from the Runtime DXE variable driver (Runtime Cache hits) and the SMM variable driver (SMM Cache hits). SMM Cache hits for GetVariable () will occur when SMM modules invoke GetVariable (). Cc: Dandan Bi <dandan.bi@intel.com> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> Cc: Eric Dong <eric.dong@intel.com> Cc: Laszlo Ersek <lersek@redhat.com> Cc: Liming Gao <liming.gao@intel.com> Cc: Michael D Kinney <michael.d.kinney@intel.com> Cc: Ray Ni <ray.ni@intel.com> Cc: Jian J Wang <jian.j.wang@intel.com> Cc: Hao A Wu <hao.a.wu@intel.com> Cc: Jiewen Yao <jiewen.yao@intel.com> Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com> Reviewed-by: Jian J Wang <jian.j.wang@intel.com> |
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