linux-kernel - Re: [PATCH v2 4/6] KVM: x86/pmu: Add pmc->intr to refactor kvm_perf_overflow{

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Date:   Fri, 10 Dec 2021 14:55:16 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Like Xu <like.xu.linux@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Kim Phillips <kim.phillips@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH v2 4/6] KVM: x86/pmu: Add pmc->intr to refactor kvm_perf_overflow{_intr}()

On Fri, Dec 10, 2021 at 1:35 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 12/10/21 01:54, Jim Mattson wrote:
> > On Thu, Dec 9, 2021 at 12:28 AM Like Xu <like.xu.linux@...il.com> wrote:
> >>
> >> On 9/12/2021 12:25 pm, Jim Mattson wrote:
> >>>
> >>> Not your change, but if the event is counting anything based on
> >>> cycles, and the guest TSC is scaled to run at a different rate from
> >>> the host TSC, doesn't the initial value of the underlying hardware
> >>> counter have to be adjusted as well, so that the interrupt arrives
> >>> when the guest's counter overflows rather than when the host's counter
> >>> overflows?
> >>
> >> I've thought about this issue too and at least the Intel Specification
> >> did not let me down on this detail:
> >>
> >>          "The counter changes in the VMX non-root mode will follow
> >>          VMM's use of the TSC offset or TSC scaling VMX controls"
> >
> > Where do you see this? I see similar text regarding TSC packets in the
> > section on Intel Processor Trace, but nothing about PMU counters
> > advancing at a scaled TSC frequency.
>
> Indeed it seems quite unlikely that PMU counters can count fractionally.
>
> Even for tracing the SDM says "Like the value returned by RDTSC, TSC
> packets will include these adjustments, but other timing packets (such
> as MTC, CYC, and CBR) are not impacted".  Considering that "stand-alone
> TSC packets are typically generated only when generation of other timing
> packets (MTCs and CYCs) has ceased for a period of time", I'm not even
> sure it's a good thing that the values in TSC packets are scaled and offset.
>
> Back to the PMU, for non-architectural counters it's not really possible
> to know if they count in cycles or not.  So it may not be a good idea to
> special case the architectural counters.

In that case, what we're doing with the guest PMU is not
virtualization. I don't know what it is, but it's not virtualization.

Exposing non-architectural events is questionable with live migration,
and TSC scaling is unnecessary without live migration. I suppose you
could have a migration pool with different SKUs of the same generation
with 'seemingly compatible' PMU events but different TSC frequencies,
in which case it might be reasonable to expose non-architectural
events, but I would argue that any of those 'seemingly compatible'
events are actually not compatible if they count in cycles.

Unless, of course, Like is right, and the PMU counters do count fractionally.