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

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Message-ID: <875z5d5x9m.fsf@nanos.tec.linutronix.de>
Date:   Mon, 07 Dec 2020 18:41:41 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.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>,
        Marcelo Tosatti <mtosatti@...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>,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

On Mon, Dec 07 2020 at 14:16, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@...hat.com> writes:
>> But other than that I don't mind making TSC offset global per VM thing.
>> Paulo, what do you think about this?
>>
>
> Not Paolo here but personally I'd very much prefer we go this route but
> unsynchronized TSCs are, unfortunately, still a thing: I was observing
> it on an AMD Epyc server just a couple years ago (cured with firmware
> update).

Right this happens still occasionally, but for quite some time this is
100% firmware sillyness and not a fundamental property of the hardware
anymore. Interestingly enough has the number of reports on Intel based
systems vs. such wreckage as obvservable via TSC_ADJUST gone down after
we added support for it and yelled prominently. I wish AMD would have
that as well.

> We try to catch such situation in KVM instead of blowing up but
> this may still result in subtle bugs I believe. Maybe we would be better
> off killing all VMs in case TSC ever gets unsynced (by default).

I just ran a guest on an old machine with unsynchronized TSCs and was
able to observe clock monotonic going backwards between two threads
pinned on two vCPUs, which _is_ bad. Getting unsynced clocks reliably
under control is extremly hard.

> Another thing to this bucket is kvmclock which is currently per-cpu. If
> we forbid TSC to un-synchronize (he-he), there is no point in doing
> that. We can as well use e.g. Hyper-V TSC page method which is
> per-VM. Creating another PV clock in KVM may be a hard sell as all
> modern x86 CPUs support TSC scaling (in addition to TSC offsetting which
> is there for a long time) and when it's there we don't really need a PV
> clock to make migration possible.

That should be the long term goal.

Thanks,

        tglx