Re: [RFC PATCH 15/41] KVM: x86/pmu: Manage MSR interception for IA32_PERF_GLOBAL_CTRL

[Jim Mattson]

On Thu, Apr 11, 2024 at 2:21 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > +     if (is_passthrough_pmu_enabled(&vmx->vcpu)) {
> > +             /*
> > +              * Setup auto restore guest PERF_GLOBAL_CTRL MSR at vm entry.
> > +              */
> > +             if (vmentry_ctrl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL)
> > +                     vmcs_write64(GUEST_IA32_PERF_GLOBAL_CTRL, 0);
> > +             else {
> > +                     i = vmx_find_loadstore_msr_slot(&vmx->msr_autoload.guest,
> > +                                                    MSR_CORE_PERF_GLOBAL_CTRL);
> > +                     if (i < 0) {
> > +                             i = vmx->msr_autoload.guest.nr++;
> > +                             vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT,
> > +                                          vmx->msr_autoload.guest.nr);
> > +                     }
> > +                     vmx->msr_autoload.guest.val[i].index = MSR_CORE_PERF_GLOBAL_CTRL;
> > +                     vmx->msr_autoload.guest.val[i].value = 0;
>
> Eww, no.   Just make cpu_has_load_perf_global_ctrl() and VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL
> hard requirements for enabling passthrough mode.  And then have clear_atomic_switch_msr()
> yell if KVM tries to disable loading MSR_CORE_PERF_GLOBAL_CTRL.

Weren't you just complaining about the PMU version 4 constraint in
another patch? And here, you are saying, "Don't support anything older
than Sapphire Rapids."

Sapphire Rapids has PMU version 4, so if we require
VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL, PMU version 4 is irrelevant.