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References: <20181119184556.11479-1-bp@alien8.de>
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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 19 Nov 2018 11:52:02 -0800
Message-ID: <CALCETrV34q-BP2w8A1s82-a8Hg062=RYzk+SCOa31W7ftsznPA@mail.gmail.com>
Subject: Re: [PATCH] x86/TSC: Use RDTSCP
To:     Borislav Petkov <bp@alien8.de>
Cc:     X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        John Stultz <john.stultz@linaro.org>,
        Tom Lendacky <Thomas.Lendacky@amd.com>
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On Mon, Nov 19, 2018 at 10:46 AM Borislav Petkov <bp@alien8.de> wrote:
>
> From: Borislav Petkov <bp@suse.de>
>
> Currently, the kernel uses
>
>   [LM]FENCE; RDTSC
>
> in the timekeeping code, to guarantee monotonicity of time where the
> *FENCE is selected based on vendor.
>
> Replace that sequence with RDTSCP which is faster or on-par and gives
> the same guarantees.
>
> A microbenchmark on Intel shows that the change is on-par.
>
> On AMD, the change is either on-par with the current LFENCE-prefixed
> RDTSC and some are slightly better with RDTSCP.
>
> The comparison is done with the LFENCE-prefixed RDTSC (and not with the
> MFENCE-prefixed one, as one would normally expect) because all modern
> AMD families make LFENCE serializing and thus avoid the heavy MFENCE by
> effectively enabling X86_FEATURE_LFENCE_RDTSC.
>

I thought I benchmarked this on Intel at some point and found the
LFENCE;RDTSC variant to be slightly faster.  But I believe you, so:

Acked-by: Andy Lutomirski <luto@kernel.org>