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

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Message-ID: <70f9a5b3-d912-2a46-3718-a9c7591cd1f4@redhat.com>
Date:   Thu, 10 Dec 2020 12:48:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     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 08/12/20 22:20, Thomas Gleixner wrote:
> 
> So now life migration comes a long time after timekeeping had set the
> limits and just because it's virt it expects that everything works and it
> just can ignore these limits.
> 
> TBH. That's not any different than SMM or hard/firmware taking the
> machine out for lunch. It's exactly the same: It's broken.

I agree.  If *live* migration stops the VM for 200 seconds, it's broken.

Sure, there's the case of snapshotting the VM over the weekend.  My 
favorite solution would be to just put it in S3 before doing that.  *Do 
what bare metal does* and you can't go that wrong.

In general it's userspace policy whether to keep the TSC value the same 
across live migration.  There's pros and cons to both approaches, so KVM 
should provide the functionality to keep the TSC running (which the 
guest will see as a very long, but not extreme SMI), and this is what 
this series does.  Maxim will change it to operate per-VM.  Thanks 
Thomas, Oliver and everyone else for the input.

Paolo