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

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Date:   Wed, 12 Dec 2018 21:00:05 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Tom Lendacky <Thomas.Lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [RFC PATCH 4/4] x86/TSC: Use RDTSCP

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.

-- 
Regards/Gruss,
    Boris.

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