linux-kernel - Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC

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Message-ID: <877dpiu29l.fsf@nanos.tec.linutronix.de>
Date:   Tue, 15 Dec 2020 23:34:46 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

On Tue, Dec 15 2020 at 07:59, Marcelo Tosatti wrote:
> On Fri, Dec 11, 2020 at 10:59:59PM +0100, Paolo Bonzini wrote:
>> So it's still true that the time advances during live migration brownout;
>> 0.1 seconds is just the final part of the live migration process.  But for
>> _live_ migration there is no need to design things according to "people are
>> happy if their clock is off by 0.1 seconds only".  
>
> Agree. What would be a good way to fix this?

None of what's proposed is fixing the underlying problem of wreckaging
CLOCK_REALTIME.

Stop proliferating broken behaviour and please answer the questions I
asked several times now:

   1) Why has the TSC has to restart at the same value?

   2) What is the technical argument that it is correct and acceptable
      to wreckage CLOCK_REALTIME by doing #1?

Thanks,

        tglx