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

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Date:   Sat, 12 Dec 2020 14:03:22 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Marcelo Tosatti <mtosatti@...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 Fri, Dec 11 2020 at 22:59, Paolo Bonzini wrote:
> On 11/12/20 22:04, Thomas Gleixner wrote:
> The nanosecond and TSC times are sent as part of the paused-VM state at 
> the very end of the live migration process.
>
> 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".

The problem is not the 0.1 second jump of the TSC. That's just a minor
nuisance. The problem is the incorrectness of CLOCK_REALTIME after this
operation which is far larger than 0.1s and this needs to be fixed.

> Again, save-to-disk, reverse debugging and the like are a different
> story, which is why KVM should delegate policy to userspace (while
> documenting how to do it right).

One ready to use option would be suspend to idle. It's fast and readily
available including all the notifications and kernel side handling of
time and whatever.

Thanks,

        tglx