linux-kernel - Re: Direct rdtsc call side-effect

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Message-ID: <98718c79-d589-3689-3c59-6ca158148641@zytor.com>
Date:   Thu, 8 Jun 2023 17:14:10 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "'Thomas Gleixner'" <tglx@...utronix.de>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     Steven Noonan <steven@...inklabs.net>,
        "kernel@...labora.com" <kernel@...labora.com>
Subject: Re: Direct rdtsc call side-effect

On 6/6/23 01:23, David Laight wrote:
> 
> IIRC the x86 performance counters aren't dependent on anything
> so they tend to execute much earlier than you want.
> OTOH rdtsc is likely to be synchronising and affect what follows.
> ISTR using rdtsc to wait for instructions to complete and then
> the performance clock counter to see how long it took.
> 

RDPMC and RDTSC have the same (lack of) synchronization guarantees; you 
need to fence them appropriately for your application no matter what.

	-hpa