Re: [RFC PATCH 4/4] x86/TSC: Use RDTSCP

From: Borislav Petkov
Date: Wed Dec 12 2018 - 15:00:18 EST

On Wed, Dec 12, 2018 at 10:50:30AM -0800, Andy Lutomirski wrote:
> As far as I know, RDTSCP gets the job done, as does LFENCE, RDTSC on
> Intel.

Same on AMD when LFENCE has been made dispatch-serializing.

> There was a big discussion a few years ago where we changed it
> from LFENCE;RDTSC;LFENCE to just LFENCE;RDTSC after everyone was
> reasonably convinced that the uarch would not dispatch two RDTSCs
> backwards if the first one was immediately preceeded by LFENCE.

Yeah, the second one won't pass the LFENCE so you won't see time going
backwards, sure.


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