linux-kernel - Re: [PATCH 3/3] x86/tsc: don't check for random warps if using direct sync

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Date:   Wed, 24 Aug 2022 16:14:49 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Cc:     Steven Noonan <steven@...inklabs.net>, usama.anjum@...labora.com,
        kernel@...labora.com
Subject: Re: [PATCH 3/3] x86/tsc: don't check for random warps if using
 direct sync

On Mon, Aug 08 2022 at 16:39, Muhammad Usama Anjum wrote:
> There's some overhead in writing and reading MSR_IA32_TSC. We try to
> account for it. But sometimes overhead gets under or over estimated.
> When we retry syncing, it sees the clock "go backwards". Hence,
> ignore random wrap if using direct sync.

This is just wrong. If the sync test can observe clock going backwards
then it can be observed during runtime too. Preventing that is the whole
point of the TSC sync exercise.

Thanks,

        tglx