Re: [PATCH v2 1/7] KVM: x86: Export kvm_pmu_is_valid_msr() for nVMX

[Sean Christopherson]

This is doing more than exporting a function, the export isn't even the focal
point of the patch.

On Mon, Nov 08, 2021, Like Xu wrote:
> From: Like Xu <likexu@xxxxxxxxxxx>
> 
> Let's export kvm_pmu_is_valid_msr() for nVMX,  instead of

Please wrap at ~75 chars.

> exporting all kvm_pmu_ops for this one case.

kvm_pmu_ops doesn't exist as of this patch, it comes later in the series.

> The reduced access scope will help to optimize the kvm_x86_ops.pmu_ops stuff
> later.

The changelog needs to explain why it's ok to add the msr_idx_to_pmc() check.

Something like:

  KVM: nVMX: Use kvm_pmu_is_valid_msr() to check for PERF_GLOBAL_CTRL support

  Use the generic kvm_pmu_is_valid_msr() helper when determining whether or not
  PERF_GLOBAL_CTRL is exposed to the guest and thus can be loaded on nested
  VM-Enter/VM-Exit.  The extra (indirect!) call to msr_idx_to_pmc() that comes
  with the helper is unnecessary, but harmless, as it's guaranteed to return
  false for MSR_CORE_PERF_GLOBAL_CTRL and this is a already a very slow path.

  Using the helper will allow future code to use static_call() for the PMU ops
  without having to export any static_call definitions.

  Export kvm_pmu_is_valid_msr() as necessary.

All that said, looking at this whole thing again, I think I'd prefer: