Re: [RFC PATCH 28/41] KVM: x86/pmu: Switch IA32_PERF_GLOBAL_CTRL at VM boundary

[Jim Mattson]

On Thu, Apr 11, 2024 at 2:54 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > +static void save_perf_global_ctrl_in_passthrough_pmu(struct vcpu_vmx *vmx)
> > +{
> > +     struct kvm_pmu *pmu = vcpu_to_pmu(&vmx->vcpu);
> > +     int i;
> > +
> > +     if (vm_exit_controls_get(vmx) & VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL) {
> > +             pmu->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
> > +     } else {
> > +             i = vmx_find_loadstore_msr_slot(&vmx->msr_autostore.guest,
> > +                                             MSR_CORE_PERF_GLOBAL_CTRL);
> > +             if (i < 0)
> > +                     return;
> > +             pmu->global_ctrl = vmx->msr_autostore.guest.val[i].value;
>
> As before, NAK to using the MSR load/store lists unless there's a *really* good
> reason I'm missing.

The VM-exit control, "save IA32_PERF_GLOBAL_CTL," first appears in
Sapphire Rapids. I think that's a compelling reason.

> And we should consider adding VCPU_EXREG_GLOBAL_CTRL so that we can defer the
> VMREAD until KVM actually tries to access the guest value.