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

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Date:   Mon, 07 Dec 2020 14:16:15 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     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>,
        Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

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). 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).

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.

-- 
Vitaly