[edk2-devel] RDTSC is not a serializing instruction

[Wu, Jiaxin]

It does has the impact, I have observed the phenomenon due to speculation execution.

Thanks,
Jiaxin

> -----Original Message-----
> From: Ni, Ray <ray.ni at intel.com>
> Sent: Thursday, May 25, 2023 2:11 PM
> To: devel at edk2.groups.io
> Cc: Ni, Ray <ray.ni at intel.com>; Wu, Jiaxin <jiaxin.wu at intel.com>; Wu, Hao A
> <hao.a.wu at 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


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