It does has the impact, I have observed the phenomenon due to speculation execution.
Thanks, Jiaxin > -----Original Message----- > From: Ni, Ray <ray...@intel.com> > Sent: Thursday, May 25, 2023 2:11 PM > To: devel@edk2.groups.io > Cc: Ni, Ray <ray...@intel.com>; Wu, Jiaxin <jiaxin...@intel.com>; Wu, Hao A > <hao.a...@intel.com> > Subject: RDTSC is not a serializing instruction > > All, > According to below Intel SDM content: > > The RDTSC instruction is not a serializing instruction. It does not > > necessarily > wait until all previous instructions have been executed before reading the > counter. > > Similarly, subsequent instructions may begin execution before the read > operation is performed. The following items may guide software seeking to > order executions of RDTSC: > > If software requires RDTSC to be executed only after all previous > instructions have executed and all previous > > loads are globally visible,1 it can execute LFENCE immediately before > RDTSC. > > If software requires RDTSC to be executed only after all previous > instructions have executed and all previous > > loads and stores are globally visible, it can execute the sequence > MFENCE;LFENCE immediately before RDTSC. > > If software requires RDTSC to be executed prior to execution of any > subsequent instruction (including any > > memory accesses), it can execute the sequence LFENCE immediately > after RDTSC. > > RDTSC might not return the accurate "current" tick. > A robust implementation could be "lfence" then "rdtsc". Otherwise, the edk2 > perf-logging infra might generate incorrect data if using > UefiCpuPkg/CpuTimerLib. > > Lots of code polling for registers within a certain timeout is also impacted. > I > am curious why "timeout" doesn't happen. Or maybe "timeout" did happen > that resulted in a longer "timeout" value was used? > > Comments? > > Thanks, > Ray -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#105491): https://edk2.groups.io/g/devel/message/105491 Mute This Topic: https://groups.io/mt/99124748/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
