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From: Andy Lutomirski <luto@kernel.org>
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>
Subject: Re: [PATCH] x86/TSC: Use RDTSCP
Date: Mon, 19 Nov 2018 11:52:02 -0800	[thread overview]
Message-ID: <CALCETrV34q-BP2w8A1s82-a8Hg062=RYzk+SCOa31W7ftsznPA@mail.gmail.com> (raw)
In-Reply-To: <20181119184556.11479-1-bp@alien8.de>

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>

  parent reply	other threads:[~2018-11-19 19:52 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-19 18:45 [PATCH] x86/TSC: Use RDTSCP Borislav Petkov
2018-11-19 19:02 ` Lendacky, Thomas
2018-11-19 19:52 ` Andy Lutomirski [this message]
2018-11-19 20:17   ` H. Peter Anvin
2018-11-19 20:40     ` Borislav Petkov
2018-11-19 20:48       ` hpa
2018-11-20  9:11 ` [tip:x86/timers] " tip-bot for Borislav Petkov
2018-11-23 20:03 ` [PATCH] " Guenter Roeck
2018-11-23 20:22   ` hpa
2018-11-23 20:44     ` Thomas Gleixner
2018-11-23 20:44   ` Borislav Petkov
2018-11-23 21:03     ` Guenter Roeck
2018-11-23 21:07       ` Borislav Petkov
2018-12-07 18:39         ` Borislav Petkov
2019-01-16 11:59 ` [tip:x86/alternatives] " tip-bot for Borislav Petkov

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