Re: [PATCH v2 17/54] KVM: x86/pmu: Always set global enable bits in passthrough mode

[Sean Christopherson <seanjc@google.com>]

On Wed, May 08, 2024, Dapeng Mi wrote:
> 
> On 5/8/2024 12:36 PM, Mingwei Zhang wrote:
> > if (pmu->passthrough && pmu->nr_arch_gp_counters)
> >
> > Since mediated passthrough PMU requires PerfMon v4 in Intel (PerfMon
> > v2 in AMD), once it is enabled (pmu->passthrough = true), then global
> > ctrl _must_ exist phyiscally. Regardless of whether we expose it to
> > the guest VM, at reset time, we need to ensure enabling bits for GP
> > counters are set (behind the screen). This is critical for AMD, since
> > most of the guests are usually in (AMD) PerfMon v1 in which global
> > ctrl MSR is inaccessible, but does exist and is operating in HW.
> >
> > Yes, if we eliminate that requirement (pmu->passthrough -> Perfmon v4
> > Intel / Perfmon v2 AMD), then this code will have to change. However,
> Yeah, that's what I'm worrying about. We ever discussed to support mediated
> vPMU on HW below perfmon v4. When someone implements this, he may not
> notice this place needs to be changed as well, this introduces a potential
> bug and we should avoid this.

Just add a WARN on the PMU version.  I haven't thought much about whether or not
KVM should support mediated PMU for earlier hardware, but having a sanity check
on the assumptions of this code is reasonable even if we don't _plan_ on supporting
earlier hardware.