linux-kernel - Re: [PATCH 0/2] RFC: Precise TSC migration

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Message-ID: <877dq1hc2s.fsf@nanos.tec.linutronix.de>
Date:   Tue, 01 Dec 2020 14:48:11 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 0/2] RFC: Precise TSC migration

On Mon, Nov 30 2020 at 16:16, Marcelo Tosatti wrote:
>> Besides, Linux guests don't sync the TSC via IA32_TSC write,
>> but rather use IA32_TSC_ADJUST which currently doesn't participate
>> in the tsc sync heruistics.
>
> Linux should not try to sync the TSC with IA32_TSC_ADJUST. It expects
> the BIOS to boot with synced TSCs.

That's wishful thinking.

Reality is that BIOS tinkerers fail to get it right. TSC_ADJUST allows
us to undo the wreckage they create.

Thanks,

        tglx