linux-kernel - Re: [PATCH v4 04/10] KVM/x86: intel_pmu_lbr

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Date:   Fri, 4 Jan 2019 07:53:36 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        like.xu@...el.com, Jann Horn <jannh@...gle.com>,
        arei.gonglei@...wei.com
Subject: Re: [PATCH v4 04/10] KVM/x86: intel_pmu_lbr_enable

On Fri, Jan 4, 2019 at 2:03 AM Wei Wang <wei.w.wang@...el.com> wrote:
>
> On 01/03/2019 11:34 PM, Jim Mattson wrote:
> > Fast forward to, say, 2021. You're decommissioning all Broadwell
> > servers in your data center. You have to migrate the running VMs off
> > of those Broadwell systems onto newer hardware. But, with the current
> > implementation, the migration cannot happen. So, what do you do? I
> > suppose you just never enable the feature in the first place. Right?
>
> I'm not sure if that's the way people would do with their data centers.
> What would be the point of decommissioning all the BDW machines when
> there are important BDW VMs running?

TCO increases as hardware ages, while the TCO for each successive
generation of hardware tends to be lower than its predecessor. Thus,
the point of decommissioning all BDW hosts that are beyond their
useful service life is to save money. Our assumption for Google Cloud
is that we will always be able to emulate older Intel processors on
newer Intel processors, so the running BDW VMs should not be affected
by the decommissioning of BDW hardware. Obviously, that means that we
won't offer features that don't have a forward migration story, such
as this one.

Yes, someday Intel will drop support for some feature that we
currently offer (like MPX, perhaps), and that will cause us some
grief.