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

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Message-ID: <F724A9C4-BA69-4713-B267-11547832F8FB@zytor.com>
Date:   Mon, 19 Nov 2018 12:48:49 -0800
From:   hpa@...or.com
To:     Borislav Petkov <bp@...en8.de>
CC:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Tom Lendacky <Thomas.Lendacky@....com>
Subject: Re: [PATCH] x86/TSC: Use RDTSCP

On November 19, 2018 12:40:25 PM PST, Borislav Petkov <bp@...en8.de> wrote:
>On Mon, Nov 19, 2018 at 12:17:35PM -0800, H. Peter Anvin wrote:
>> On 11/19/18 11:52 AM, Andy Lutomirski wrote:
>> > 
>> > 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@...nel.org>
>> > 
>> 
>> As long as the difference isn't significant, the simplicity would
>seem to be a
>> win.
>
>Right, I think by simplicity you mean RDTSCP. :)
>
>Also in the sense that you have a single instruction which gives you
>that barrier of waiting for all older insns to retire before reading
>the
>TSC vs two where you don't necessarily know what happens on the uarch
>level. I mean, nothing probably happens but RDTSCP is still simpler :)
>
>Also, hpa, isn't LFENCE; RDTSC and RDTSCP equivalent on Intel? In the
>sense that RDTSCP microcode practically adds an LFENCE before reading
>the TSC?
>
>Because SDM says:
>
>"The RDTSCP instruction is not a serializing instruction, but it does
>wait until all previous instructions have executed and all previous
>loads are globally visible."
>
>which sounds pretty much like an LFENCE to me:
>
>"LFENCE does not execute until all prior instructions have completed
>locally, and no later instruction begins execution until LFENCE
>completes."

I don't know the exact sequence of fencing operations it is equivalent to, but it is probably something quite similar.
-- 
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